Create the device manager during the event initialization, where it
makes sense.
This allows us to get rid of the per-backend get_device_manager()
virtual function, and just store the DeviceManager pointer into the
ClutterBackend structure.
Instead of piggybacking on the EGL backend, let's create a small
ClutterBackend for the CEx100 platforms. This allows us to handle the
CEx100-specific details in a much cleaner way.