mainview: UI call management refactor

- AccountAdapter::navigateToWelcomePageRequired is no longer required since subscription to property change in QML
- avoid unnecessary CallAdapter signals for UI management. Listen to BehaviorController and avoid duplicated / unnecessary signals.
- avoid duplicated code:
  - account selection only in AccountAdapter::changeAccount and ConversationsAdapter::selectConversation
  - simplification of ConversationsAdapter::selectConversation
  - conversation UI always managed by ConversationsAdapter -> ConversationSmartListItemDelegate
- smartlistmodel: no need to show callstack for outgoing calls in state ENDED

Gitlab: #86
Gitlab: #87
Gitlab: #88

Change-Id: I7025d4292914939f5a78aee1c4742104b370641d