mainapp: dynamically load views

Introduces the ViewCoordinator component to promote dynamic view loading and reduce coupling between components.

The following objects are now created and destroyed as needed:
- SettingsView
- WizardView
- dialogs

Further refactoring will be required in order to do the same with the ConversationView, which now parents the ChatView and the CallStackView.

Gitlab: #897
Change-Id: Ice6a0c133e62e1e0c8d7fb99ec2c41234c049b59