ui: shift+direction extends text highlighting

It should behave like every other text box implementation:
If the cursor is a point,
* shift+{up,left} change it into a selection, extending the beginning
* shift+{down,right} change it into a selection, extending the end

If the cursor is a selection, one end of it is the extension point.
* If the selection was created from the mouse, the extension point is the end that does not correspond to where the mouse first clicked. (In other words, it is the end of the selection corresponding to unclick or to the drag point if the mouse was not yet unclicked).
* It the selection was created from the keyboard (as above), the extension side is as described above.

shift+direction moves the extension side in the corresponding way.