Bug 599573 - Remember dialog defaults between Gimp sessions

Add GimpFillOptions and GimpStrokeOptions to GimpDialogConfig and use
them in the Fill/Stroke Selection/Path dialogs and for the "with last
values" commands. Add GUI for them to Preferences -> Dialog Defaults.

This requires most of the stuff in my last few commits, and some
more changes:

GimpFillOptions is a GimpContext which has all sorts of connections to
everything, including a Gimp pointer. Hack around in GimpDialogConfig
to add a Gimp property, and add "gimp" parameters to quite some GimpRC
functions. Treat the Gimp* as a GObject* in all public API because
core/ stuff is not known in config/.