Use new libhandy widget to lay out the Recent/Contacts/Dial pad parts of the main window

Related features: https://source.puri.sm/Librem5/use-cases/issues/106 #53 (closed)

At the moment there is a back button and a plus button to switch between the recent calls like and dial pad. This should be changed to use the new libhandy widget which offers a stack-switcher style interface.