app: Chain-up correctly

Fix a typo which made us chain-up to the wrong method of base class.
machine: Remove redundant timeout

Show the display immediately on going to DISPLAY UI state.
app: Seperate out main window

Separate out Gtk.ApplicationWindow related code from App into another
class, AppWindow and put its UI setup to a .ui file.

I regret not being able to do this in smaller patches but I bumped into
many unexpected issues on the way, making it extremely difficult to
divide the changes afterwards.