Sort out a bit of confusion between mouse- and keyboard-driven drags

Sort out a bit of confusion between mouse- and keyboard-driven drags
in Rectangles. Mouse drags now take priority - you can't start a
keyboard drag while the mouse is held down, and starting a mouse drag
instantly cancels an unfinished keyboard drag - and also I've fixed an
assertion failure which would come up if you had the keyboard cursor
visible at the end of a mouse drag (by pressing arrow keys while the
mouse was held down).

[originally from svn r9393]