Removing meaningless distinction between NULL pointer and EVT_TABLET_NONE,
and initialize pressure and tilt to 1.0 and 0.0 respectively when no tablet
is used.
event_system.c has been collecting a lot of different functionality,
move generic query/access functions into their own file,
since these are used by operators and other parts of the windowing code
and aren't part of low level event handling.
Also move public last-property API to wm_operators.c.
