Add an enum for gestures that GtkShortcutsShortcut knows how to show an image and subtitle for. https://bugzilla.gnome.org/show_bug.cgi?id=758187
This is a toplevel window that is tailored towards showing help for shortcuts in an application. The implementation closely follows this design: https://wiki.gnome.org/Design/OS/HelpOverlay This implementation is inspired by earlier work in gnome-builder, thanks to Christian Hergert. https://bugzilla.gnome.org/show_bug.cgi?id=756428