New keyboard handler for play commands

[oscarlorentzon]

The following keyboard handlers that were not implemented in #234 could be added:

Play/Reverse Play/Stop
Pan
Reset view to zero zoom and center at [0.5, 0.5]

[oscarlorentzon]

Adding the following commands to this issue:

Increase/decrease play speed
Switch playing direction

Removing panning related handlers from the scope of this issue.