Fix phantom retriggering when using Novation Launchpad

[jp-bennett]

Running Ardour with "-D trigger" revealed that the Novation Launchpad Mini has a habit of sending multiple pad_press events for a single press. These extra phantom presses were getting stuck in the explicit_queue. Rather than a complicated debounce algorithm, this code just pops the event off the queue when it's triggering the same box multiple times.