Inversion of Page.selectionStart/selectionEnd when selecting backward

[CyrilleB79]

In 6pad++, the member selectionStart and selectionEnd of the Page class are always returned so that selectionStart is lesser selectionEnd.
The documentation states that selectionStart is the anchor point of the selection, i.e. the point where the selection action has been started and that remains fix during the action.
Thus, when selecting backward, selectionStart/selectionEnd should be returned so that selectionStart be greater that selectionEnd.