evdev: Associate devices with the first created stage

evdev: Associate devices with the first created stage

Because evdev isn't associated with the display system, it doesn't
have any easy way to associate an input device with a stage.
Previously Clutter would never set a stage for an input device and
leave it up to the application to set it. To make it easier for
applications which just have a single fullscreen stage (which is
probably the most common use case for evdev) the device manager now
associates all input devices with the first stage that is created
unless something has already set a stage.

Reviewed-by: Emmanuele Bassi <ebassi@linux.intel.com>