Every situation where the user disables a property through the inspector should
now correctly unbind any dependent bound properties, which fixes the last known
major bug in the property binding code!

This is supposed to be signalled whenever a property gets invalid,
e.g. when switching a GtkLabel to use a pixbuf instead of a stock
icon. Binding target properties use this to remove any property
bindings that become invalid this way.

The signal is emitted when glade_property_invalidate() is called.
Currently, the GladeEntryEditor does this, so binding a property
to an entry's "Text" and then changing the entry to use a "Buffer"
correctly removes the binding. More callers will follow.

Later, object property value resolution could go here, too. For you, Juan! :)

Juan, Tristan and I agreed that the GTK+ side of this is not ready yet.
The code will be moved to a new "gbinding-tranform" branch, though, for
eventual later integration.

I changed glade_project_fix_props() back to glade_project_fix_object_props()
but did not adjust the call to it.

For Juan. :)

As to Tristan's request. This makes all the signal handling hackery I did
in glade-property.c obsolete, which greatly simplifies the code! The only
drawback is that changes of a binding source property's "enabled"/"sensitive"
state are not recognized anymore as these are not registered as commands.
How this will be fixed will still have to be discussed wih Pablo and Tristan
(likely after GSoC).

Now, when the binding source of a property is disabled, the property
is reset to its default value and unbound. Enabling the source again
rebinds the property. Such "dead" property bindings are not saved
and thus only persist as long as Glade is opened. I hope this behavior
is in line in what users would expect; if not, it's easy to change.

There is now a new checkbutton in the "Bind to source..." dialog to set
a transformation for a binding. Because we don't have the transformation
function and the source and target properties might have different types,
there is no in-workspace preview of the binding; instead, a property with
a transformed binding is shown as having its default value.

Instead of g_object_set_data() kludges, the code now uses a newly introduced
GladeBindingDialog struct to store values later needed by the dialog's signal
handlers. This cleanup helps with the introduction of transformation functions.

This took me the whole night to triage and fix... nasty subtleties
revolving around the GladeProperty finalization process, including
signal handlers that were called after the relevant objects were
destroyed... much fun.

Select the current binding source in the "Bind to source..." dialog if the target property already has one

As the function now also "fixes" bound properties, this is a more accurate name.

This complements the introduction of the new "transform-func" attribute
for <binding> tags that I recently push to my gtkbuilder-gbinding GTK+
branch. Glade can now open files with such attributes and save them again
without data loss.

This also adds a new "binding-transform-func" property to GladeProperty.

How senseless would that be?...

This was a side object of reusing the already existing
glade_eprop_object_object_view() function. This is not
my most beautiful work, but it works reasonably well and
the code wasn't that pretty to begin with. (I know, lame
excuses...)

The code is carefully written to resurrect such property bindings if the
widget removal is undone.

Rename the OK button in the "Bind to source..." dialog to "Bind" and make it only sensitive if a compatible source property is selected

In the binding source chooser dialog, show uncompatible properties greyed out beneath the compatible ones

This is less confusing than hiding incompatible properties all together.

The dialog now has a two-pane layout, with a  left pane to choose
a source object and the right to choose a property of that object.
The property list is filtered to show only properties whose type
is compatible with the target property.

This makes it possible to rebind to another property.

Add "Unbind" context menu item for properties and disable "Set default value" for properties which are bound

As discussed with Tristan van Berkom. This fixes races that formerly existed
between creating a binding and attaching it to a GladeProperty (in this timeframe,
two GladeBindings were active at once) and - surprisingly - makes the code quite a
bit cleaner. This should be a much better base for implementing the next features
on my TODO list than the former one was.

As acherry on top, the new structure allowed me to fix a bug on the way: when
undoing a property binding action, the binding target property was editable
again, but the binding itself didn't actually disappear (meaning that editing
the source property still overrode the value of the target). Now, everything
works as intended. :)

Implement "Bind property" command to undo/redo system and expose it as "Bind to source..." popup menu item in the property inspector

Don't take the UI of the "Bind Property" dialog seriously, it's just for testing purposes. Instead of raw text entries, there will naturally be a list of widgets and properties to choose from or something. However, the actual commands,
including undo/redo, do already work.