Add/clear block queue

makes a number of changes utilizing the ability to flush the block queue...

abort now stops almost immediately, and tried to set the last milestone to actual actuator position.
Allow definition of a kill button (or use pause/play as kill) this will stop immediately most of the time
Above uses ON_HALT which no longer uses the pauser.
Send !! to host when in halted state to stop streaming
M999 will clear the halt status, however positions will be wrong and homing will be required
Hitting an enabled limit switch uses ON_HALT too, and can be reset using M999, it should also allow jogging off the limit switch
Fast flashing of play/pause led means in halt state
Panel updated to show halted state, and a menu item to clear it
Added realtime actuator position tracking, needed to do FK to find position after an abort.

Some general cleanup to try to save more RAM and offset the small amount of RAM these changes added.