TextPane: shift-cursor selection behaviour doesn't match other editors when moving backwards and forwards

1. Click somewhere in a paragraph.
2. Hold down the shift key.
3. Press right-arrow twice.
Two characters are now selected.
4. Press left-arrow once.
Now 3 characters are selected.

The expected behaviour after step 4 is that only character is selected.

The tricky part about this is that it requires us to keep track of where the selection process _began_, because we need to know if the user is moving forwards or backwards from the original selection position.