Code reorganization

Currently, the file player.py contains much of the application logic, like the definition of the class of the main object (Player, which is an instance of QMainWindow and is instantiated in command.py). Much of the code in player.py should be put into a separated file, say app.py. All the code related to the control of the video player should be put into a new file video.py (after changing in terminology Player ⇒ Video, as per issue #95 (closed). Also, the key events should be moved into the future TimePane class (as per issue #88 (closed)).

The widget.py, which has, currently, 1409 lines(!), should be broken down into separate files, one for each widget (TimePane, Timeline, Annotation, Event, and Cursor).