Fixes the replotting update. It's now based on a QTimer so it's in the event buffer...

Fixes the replotting update. It's now based on a QTimer so it's in the event buffer as opposed to a best effort based on a hand-made timer. The interval between GUI updates is settable through the qtsink_X objects with set_update_time(newtime). This update makes the plotting much more stable. It also fixes the time scale of the waterfall plot to actually mean the right thing.