Add a seperate camera for play mode

Play mode now has a seperate entity to represent the camera. This is
automatically added to the scene if it doesn't already exist after
loading a file. The camera is not internal so it can be animated,
serialized and saved. The RigCameraView now uses this camera instead
of its view camera when in play mode. The entity has a handle entity
so that it can be selected and will be visible in the scene. However
it's currently quite difficult to find it because it is usually
positioned far away from the scene.
Reinstate the Ctrl+C key to select the camera

The Ctrl+C key used to select the view camera so that its properties
could be manipulated. This was removed since the switch to a separate
camera view in bf635357 made this difficult. This patch brings the
keybinding back but makes it select the global play camera instead of
the view camera. This is useful because it's not always easy to find
the handle for the play camera if it is far away from the scene.