Add input queue to backend, fixing process leak problem.

Add input queue to backend, fixing process leak problem.

Mouse and keyboard input is handled immediately, all other input is put on a
queue which is processed whenever gui_mch_update() is called.  This avoids DO
calls to be sent from the backend during processing of another DO call.  (See
comments in MMBackend processInput:data: and processInputQueue.)  One problem
this caused was that connectionDidDie notification was not received when
processInput:data: got called recursively (and thus processes could "leak").

Also did some code cleanup to MMBackend.