Niels De Graef authored May 15, 2017 and Michael Gratton committed Oct 18, 2017

* Ported Gtk.Action to GLib.Action in the GearyController.
* Removed Gtk.AccelGroups (handled through Gtk.Application now).
* Got rid of Gtk.UiManager (now all is done through Gtk.Builder).
* Throw away workaround for conflicting Gtk.Actions in ComposerContainer.
* Aggregate zoom in/out/normal into one parameterized zoom action.

Signed-off-by: Niels De Graef <nielsdegraef@gmail.com>