Niels De Graef authored Aug 22, 2016 and Michael Gratton committed Sep 21, 2016

Now using these instead of the old composer's actions. This led to quite
some changes:

* Use GLib.ActionEntry instead of Gtk.ActionEntry in
  composer-widget.vala
* Action names can now be specified in the UI files.
* Use templates for the ComposerHeaderBar. Remove
  Pillbar as superclass, since that was no longer necessary.
* Merge ComposerToolbar into ComposerWidget.
* Since actions can now be parameterized, some methods could be
  merged (e.g. font size methods).
* The menu button in the composer now automatically uses a popover.
* Some methods and classes really deserved more comments.
* necessary POTFILES.in changes

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