Support visual cursor movement in bidirectional text

Qt currently moves the cursor through bidirectional text in a logical order. This is the same as the behavior on Windows, but on Mac and some other platforms, the cursor movement is purely visual.

On platforms where it is the expected behavior, Qt should support visual movement of the text cursor. This requires some investigation as to what can be done in cases where the cursor has two visual positions (between a RTL and LTR character), and what should be done for keyboard selections.