Use Q_GLOBAL_STATIC

It's way better to make singletons.

Issue: #23
Set initial geometry

VCompositor API slightly changed, now an initial geometry is passed to
the compositor when it creates the output window.
QWindow-based compositor is now called "classic"

Main goal of this compositor is to be a simple desktop compositor
without too much animations and effects.
One day we might think about writing a specific shell.

We keep this compositor temporarily disabled until it has been
ported to the new QtCompositor API.
Initial desktop_shell interface implementation

Start writing code to implement Weston's desktop_shell interface.

Issue: #23
QML-based desktop shell becomes "desktop" again.

Rename the plugin to "desktop".
Fix shell surfaces creation

Move shell surfaces creation to WaylandIntegration and don't set user
data to the object that creates the window otherwise QPA plugin will
crash because it expect a QWindow. The wayland QPA plugin already sets
the user data to QWindow, though.
We also don't want to listen to Wayland events anymore since outputs
are already handled by QPA.