Support GtkShortcutsWindow and constituents
@matthiasc
Submitted by Matthias Clasen Link to original bug (#758519)
Description
I've pushed a wip/matthiasc/shortcuts branch which adds support for GtkShortcutsWindow, GktShortcutsSection, GtkShortcutsGroup and GtkShortcutsShortcut.
The branch depends on API additions and fixes that landed in GTK+ master after 3.19.2