mainapplication: isolate window mode logic in a top-level component

Logic for fullscreen/windowed mode switching is scattered within
the main ApplicationWindow and other components. Ideally,
components that need to transition to fullscreen, can make a dumb
request to module in charge of making the necessary checks and
carrying out the mode change.

This patch introduces the LayoutManager used to isolate this logic.

Change-Id: I0e5b932617d2b88eda1533f25a5d55fc1c66c438