Change annotation border position with the mouse

Presently, when an annotation is double-clicked, rectangle handles appear at its borders. When clicking on a handle, the timeline cursor position is changed to coincide with the position of the clicked handle. The position of the handle can be changed with the left and right arrow keys, and the position of the cursor follows the position of the handle. For the sake of coherence, clicking and dragging the cursor should also allow the changing of the handle position.

This change in behavior should probably be done together with the fix of issue #5 (closed).