Add a modifier button to grab or pick all actors in the scene in VR

This adds a modifier state that can be mapped to a controller button.

When the modifier is active (the button is pressed), the grab/pick action picks all actors instead of only the one that is picked. This allows moving them simultaneously. Given that the pick/grab action is mapped by default to the right trigger, the modifier button is mapped to a button on the left controller (depending on the controller type).