Refactor|Input: Fixed a boolean trap in the input subsystem
Client|API: Querying if a control has bindings, added new look controls

The public Player API was enhanced with a function for querying
whether a control has any bindings.
libcommon: Added lookpitch/lookyaw controls

"lookpitch" (CTL_LOOK_PITCH) overrides the delta-based CTL_LOOK if
it has bindings.

Also did some cleanup.
Input: Generate some test events for the Head Tracker virtual device

This can be tested with the binding "bindcontrol lookpitch head-pitch".

If the binding exists, normal mouse Y etc. bindings will be ignored
and the absolute angle from 'head-pitch' is used instead.

(Also helpful for testing: 'listbindings', 'delbind')