Allow to unset the link mode when clicking the unlink button at one end of the link text.

Currently, the behaviour of the unlink command is the following: if the cursor is at one end of the link text but inside the link and the user clicks the unlink button, the link will be removed from the whole text. The desired behaviour is that, if the cursor is at one end, the unlink button just unsets the link mode from the editing so that she can start typing normal text.

Note that a consequence of the current behaviour is the impossibility of the user to write text outside a link at the end of a document (because the cursor is inside the link and he cannot unset that).