Include code in midend.c to sanitise streams of mouse events so that

Include code in midend.c to sanitise streams of mouse events so that
back ends can be assured of always receiving them in a sensible
sequence (button-down, followed by zero or more drags, followed by
button-up, and never changing button in the middle of such a
sequence). I have a suspicion this issue was the cause of the
mysterious Pattern grid updates seen by Dan during testing last
night.

[originally from svn r5208]