Markdown formatted.

The value arrays first value is the encoded x-default. Use that in the
UI to manipulate.

My first implementation of converting string values to "raw" values
would fail if the user would use characters I'm splitting on. This
implementation should be more implementation independant.

With libgexiv2, we get preprocessed data. Having two columns storing
different information (scalar and raw values) makes it hard to keep the
data consistent.
Now the [set|get]_scalar methods can be used to store values and act as
accessor and getter methods. The underlying set|get methods will
automatically set the corresponding scalar string values anyways.

Becase we'll use libgexiv2 to retrieve scalar string values, we'll most
likely have to preprocess some of them in order to correctly
store/edit/export them again. This patch is based on a hunch with the
integration and might not work out as expected.

Since we're replacing the parser by gexiv2, this tool isn't needed anymore.

This workaround allows to run the plug-in when opening a JPEG. The
import/merge of EXIF data is currently not working. All data is set as a
scalar property. This is invalid data by the XMP metadata
standard.  A proper fix will need more code for the import in
exif-decode.c

This reverts commit 5491b567135aeeea83ab943450a1ab9709ef1e32. The reason
for that is, that we can use a
gtk_tree_view_insert_column_with_data_func to setup the treeview instead
of using signals.

This will break the detailed signal subscription and lead to not
updating widgets during import.

According to the documentation the strings are freed automatically.

If the user changes values in the treeview, we don't want that the
widget updates the XMPModel again.

Instead of rendering the icons for the advanced view from this widget,
we keep this responsibility in the interface (in the treeview). This
also revealed a bug the icon to disappear because of circular signals
being handled/fired.

Once the tree view is focused by the user, we update the icons. This is
currently only necessary, because the XMPModel functions overwrite the
icon once the user changes a property. An update function should handle
this in the future to make this perhaps obsolete.

If the user changed sequence properties in the UI, we need to split up
the changed string representation or use the raw value otherwise.

Currently we're using a text widget. Perhaps a tag widget is more
suitable to enter keywords.

The description writer is a XMP_TYPE_TEXT property, which should be
serialized correctly._

Correctly serialize XMP Metadata in case the user creates an empty image
and attaches metadata to it.

Replaced test, which assures that values set on an empty model are also
correctly serialized.

This handler now correctly serialises data set in the entry widgets.