cursor: Add an explicit getter for the gbm device

We want to move this to a new MetaCursorRenderer class.
Split out the code that renders the code into a MetaCursorRenderer

Right now, this is the same exact same mess it's always been, but
it will be fixed up soon with backend-specific renderers.
monitor-manager: Remove debugging META_DEBUG_MULTIMONITOR hook

It's outdated and incorrect: META_TYPE_MONITOR_MANAGER is now an
abstract class.
idle-monitor: Use the XSync idle monitor under X11 nested
backends: Start on an initial MetaBackend object

This isn't great so far -- all we did is put the idle monitors here
instead. We'll soon have separate backend subclasses for the two
backends.
backends: Build MetaBackend subclasses for each backend
backends: Move most of the code from meta_backend_init into the subclasses
backends: Move MonitorManager creation to MetaBackend as well