Fix occurrence borders management

There are still some problems:

• When the cursor is positioned one frame after the right border of an occurrence, the borders of this latter are thickened. They should not.
• When creating a new occurrence, its right border can go past the left border of the next occurrence. It should not.
• Starting a new occurrence when the cursor is positioned at the frame after the right border of an occurrence is impossible. This should not be the case.

The right way of fixing these issues is by changing the internal representation of time in both TimePane an Occurrence classes. Instead of using float numbers, we should use integer numbers, corresponding to the frames in the video.