CallWindow: First shot at GTK3 porting.

- We now use a GtkHeaderBar with icons and a GtkBuilderMenu.
- Ekiga::Call is now an Actor:
  * Most actions are now dynamically added thanks to GActorMenu
  * We do not need to sensitive/unsensitive menus and toolbars items
    following the call progress events. The Ekiga::Call implementation
    is taking care of adding and removing Actions dynamicall. The GIO
    stuff will sensitive/unsensitive things accordingly.
  * The call transfer popup is now implemented by the Ekiga::Call
    implementation in two lines of code. That simplifies our code.

This is still partial code, but already usable.