When filling in or blanking a square, don't generate null moves

This applies to various square-filling games: Keen, Solo, Towers,
Undead, and Unequal.  In all cases, selecting a square and pressing the
number that was already in it, or selecting an empty square and pressing
Backspace, would add a move to the undo chain that did nothing.  This
also meant that the convention where Backspace from the top level of an
application in KaiOS leaves the application didn't work.

Now the various interpret_move() functions check the current state of
the grid, and return NULL or UI_UPDATE where a move wouldn't change the
board.  UI_UPDATE is returned in the case where the cursor was put in
place using the mouse, because in those cases I think the cursor should
still be hidden again.  NULL is returned when the cursor was put in
place by keyboard, because then there's really nothing to do.

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