callview: use dynamic loading for call views

This commit replaces a StackLayout with a Loader allowing us to load initial and ongoing call views dynamically based on the current conversation's call state.

This may fix several issues related to conversation loading including a possible uncaught binding loop based on observing CurrentConversation.id changes.

- small header clean up

Change-Id: Idfc723d8b39f19aafb026c19f26590910b5c26cd