mainapplication: add URI start option

+ Refactors some window management logic in LayoutManagar.qml.
+ Refactors app start to support both a minimized start and a start
  URI. The start URI should force visibility.
+ StartMinimized now starts the application in a closed-to-tray
  state, instead of a minimized state.
+ The close-to-tray feature now saves the previous windowed state.
+ InstanceManager handles URI transfer to secondary instances.

This commit does not implement URI handling. The original
implementation supported only calls via a user infohash as the URI
parameter. A new and flexible protocol should be defined to support
multiple Jami features.

Gitlab: #655
Change-Id: I2c47028930e4e9d7ccca94d9362545df14b98160