startupfeedback: Change to be window based and controlled by a model

This makes the startup feedback more robust, by having instances be controlled by a model which can listen to window changes. Being window based also allows for the close button and gestures to work properly with it, since it will show up in the task switcher as well.

Fixes:

• #357 (closed)
• #338 (closed)
• #335 (closed) (dark themes now tint the background color)
• #330 (closed)
• #30 (closed)

TODO: add animation to status bar and nav bar to preserve colour