The gesture functionality was taken over by GtkShortcutsShortcut,
so this widget is no longer needed, and it never was in a stable
release, so lets get rid of it.
The prime example for direction-dependent shortcuts is using
<Alt>Left or <Alt>Right to go back. Support this by adding a
direction property to GtkShortcutsShortcut, and filtering by
the current text direction.
https://bugzilla.gnome.org/show_bug.cgi?id=757888
