Migrate to GTK+ 3

[Emilien Klein]

https://developer.gnome.org/gtk3/stable/gtk-migrating-2-to-3.html

Currently the code depends on GTK+ 2, and glade-gtk2 messes up the ui code, making it unusable after having saved it.
glade (the GTK+ 3 version) refuses to open the project, with an error message saying that the nautilus_image_manipulator_dialog catalog is missing...

[Emilien Klein (emilien-klein)]

Changes made to r201 make it possible to open the ui project with glade-gtk2, but once you save the ui file you won't be able to run the application anyomre, as it has removed a few bits...
In that revision I've had to make changes from GtkBox to GtkVBox and GtkHBox, which will probably have to be reverted when migrating to GTK+ 3...

[Emilien Klein (emilien-klein)]

Actually it was more of a question of being able to open the ui files with Glade for GTK+ 3. This was done in r207 and r210. It looks like everything is working fine with GTK+ 3, great!