Simplify management of mouse clicks and keystrokes

For now, there is a ping-pong among the Video, the TimeScale and the TimePane objects, as regards the management of mouse clicks and keystrokes.

The class TimeScale respond to mousePressEvent. The user clicks on the timescale in order to change the position time of the video. This triggers this chain of events:

1. TimeScale.mousePressEvent computes the time (in ms, as an integer) associated with the position clicked and passes it to Window.set_video_position.
2. Window.set_video_position just passes its argument to Video.set_position.
3. Video.set_position truncates the position to a multiple of self.mfps and passes the result to Window.set_position.
4. Window.set_position just passes its argument to TimePaneView.set_position.
5. TimePaneView.set_position does three things:
   • If there is an occurrence under creation, it calls method get_time_from_bounding_interval for that occurrence. Essentially, this method restrains its time argument to the feasible interval and returns the new bounded value.
   • Do the same thing for the selected item, if it is an occurrence.
   • Call the method Window.set_media_player_position, passing as argument the value of time (after casting it to int, unnecessarily, btw).
6. Window.set_media_player_position passes its argument position to Video.media_player.setPosition (after casting it to int, unnecessarily, btw)
7. Changing the MediaPlayer position triggers the call to method Video.position_changed, which:
   • updates the Timestamp (via Timestamp.update);
   • passes its argument to Window.position_changed.
8. Window.position_changed does two things:
   • Set the value of TimePane.time. This triggers a call to TimePane.on_time_changed
   • Call TimePaneView.update
9. TimePane.on_time_changed does this:
   • If there is an occurrence under creation, limit the time to the borders (this will trigger again TimePane.on_time_changed, eventually)
   • If there is a selected occurrence, calls Occurrence.get_time_from_bounding_interval.
   • Set the x position of the Cursor object
   • If there is an OccurrenceHandle object focused, calls its OccurrenceHandle.change_time, passing the time argument to it.
   • Update the occurrence borders, via TimePane.occurrence_borders.
10. OccurrenceHandle.change_time calls either Occurrence.update_begin_time or Occurrence.update_end_time.
11. Finally, both Occurrence.update_begin_time and Occurrence.update_end_timecall nobody.

[See issue #122 (closed).]

This must be improved, probably in connection with issues #116 (closed) and #145 (closed).