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/.