device-manager: Add private interface to manipulate platform event data

This normally belonged to the ClutterBackend, however there's device
managers (eg. evdev) that are somewhat detached from the backend, so
need to bridge this somehow.

This allows device managers to implement these bits that were usually
responsibility of the ClutterBackend.

https://bugzilla.gnome.org/show_bug.cgi?id=758238