mainWindow: Adaptive UI

When the window gets narrower than 700px, most of the buttons in the
headerbar move to a new actionbar at the bottom of the window. This
allows the window to continue to shrink, down to 300px.

The rest of the UI is not adaptive yet, just the main screen.
1 job for adaptive-ui in 12 minutes and 22 seconds (queued for 2 seconds)