Many boundary task editor improvements.

- start/end fields are stepped at 1/20th of second (0.05) instead of 1/100th (0.01).
- Entirety of possible media containing sentence is loaded.
- Only the audio between start/end plays during playback.
- Keyboard shortcuts reduced to remove use of 'ctrl' and use 's' instead of 'b'.
- start/end fields are focused when 's' or 'e' are used.