The DeviceManager class should be abstract in Clutter, and implemented
by each backend, as different backends will have different ways to
detect, initialize and list devices; the X11 backend alone has *two*
ways of dealing with devices.

This commit makes DeviceManager an abstract class and delegates the
device initialization and enumeration to per-backend sub-classes.

The responsible for creating the device manager is, obviously, the
backend singleton.

The X11 and Win32 backends have been updated to the new layout; the
Win32 backend has been updated blindly, so it might require additional
testing.