Give conversation's actions properties

[Danielle Foré]

This is mostly meant to be used to deprecate the use of Gtk.Action

Instead of having setters in geary-controller to control the sensitivity of the reply, forward, archive, etc buttons we should give messages properties like "can_reply" and then property bind the widget sensitivity. This way we can avoid making the widgets public, we only need the conversation's properties to be public and the widgets "subscribe" to that. We can remove a ton of setters from geary-controller and have the sensitivity binding be more obvious by putting it with the widget itself. We don't need to depend on Gtk.Action to broadcast the action's sensitivity, the code is easier to understand, we save some lines from geary-controller, and everyone wins.