pdInput: reduce frequency of cursor handling

By setting the class long for cursors to NULL prior to requesting cursor
changes, we can prevent DefWindowProc from attempting to reset the class
cursor definition every time the mouse is moved.
pdInput: fix tooltips disappearing during custom MouseMove handling

See comments for details; basically, marking the message as Handled
(presumably?) prevents the tooltip from receiving subsequent
notification of mouse events.  As such, we must return False even when
MouseMove messages have been handled.