feat: integrate the tray app with RPC (#19)

- Modularizes the tray window to use multiple Pages for the three
different states: signed out, disconnected from RPC, normal
- Adds the two new aforementioned pages
- Adds a CredentialManager service for storing credentials in Windows
Credential Manager
- Adds a RpcController service for managing the connection to the
backend service and reporting state changes to the ViewModel
- Switches to using separate ViewModels in the Views
- Switches to using Dependency Injection for instantiating Views,
ViewModels and Services
- Integrates the tray window into the new RpcController on Start/Stop
interactions. Workspace agent updates will be handled in a separate PR

Relates to #5 (I will do agent updates immediately after this)

Author: deansheather