QML: Improve window management

* Move some logic to compositor.js
 * Add a notification area
 * Add a fullscreen window container, which is above status bar and notification area
 * Put two buttons in DummyWindow as PoC, to switch between window states and to add a widget to the notification area
 * Notification area now has three states: closed, minimized, open
 * DPI has the same semantic as luna-next (values are expressed for DPI=132)
 * GestureArea looks better, and can be used to minimize an app
 * DummyWindow can add notifications

Signed-off-by: Christophe Chapuis <chris.chapuis@gmail.com>