Expose the NO_EFFECT/UNUSED distinction through midend_process_key()

This removed the "handled" pointer and instead extends the existing
boolean return value (quit or don't quit) into an enumeration.  One of
the values still quits the program, but now there are different values
for keys that had an effect, had no effect, and are not used by the
puzzle at all.  The mapping from interpret_move results to process_key
results is roughly:

move string    -> PKR_SOME_EFFECT
MOVE_UI_UPDATE -> PKR_SOME_EFFECT
MOVE_NO_EFFECT -> PKR_NO_EFFECT
MOVE_UNUSED    -> PKR_UNUSED

The mid-end can also generate results internally, and is the only place
that PKR_QUIT can arise.

For compatibility, PKR_QUIT is zero, so anything expecting a false
return value to mean quit will be unsurprised.  The other values are
ordered so that lower values indicate a greater amount of handling of
the key.

devel.but

@@ -3291,8 +3291,7 @@ call to this function. Some back ends require that \cw{midend_size()}
 
 \H{midend-process-key} \cw{midend_process_key()}
 
-\c bool midend_process_key(midend *me, int x, int y, int button,
-\c                         bool *handled);
+\c int midend_process_key(midend *me, int x, int y, int button)
 
 The front end calls this function to report a mouse or keyboard event.
 The parameters \c{x} and \c{y} are identical to the ones passed to the
@@ -3330,16 +3329,33 @@ Calling this function is very likely to result in calls back to the
 front end's drawing API and/or \cw{activate_timer()}
 (\k{frontend-activate-timer}).
 
-The return value from \cw{midend_process_key()} is \cw{true} unless
-the effect of the keypress was to request termination of the program.
-A front end should shut down the puzzle in response to a \cw{false}
-return.
+The return value from \cw{midend_process_key()} is one of the
+following constants:
 
-If the front end passes in a non-NULL pointer in \c{handled}, the
-mid-end will set \cw{*handled} to \cw{true} if it or the backend does
-something in response to the keypress.  A front end can use this to
-decide whether to pass the keypress on to anything else that might
-want to do something in response to it.
+\dt \cw{PKR_QUIT}
+
+\dd Means that the effect of the keypress was to request termination
+of the program.  A front end should shut down the puzzle in response
+to a \cw{PKR_QUIT} return.
+
+\dt \cw{PKR_SOME_EFFECT}
+
+\dd The keypress had some other effect, either in the mid-end or in
+the puzzle itself.
+
+\dt \cw{PKR_NO_EFFECT}
+
+\dd The keypress had no effect, but might have had an effect in
+slightly different circumstances.  For instance it requested a move
+that wasn't possible.
+
+\dt \cw{PKR_UNUSED}
+
+\dd The key was one that neither the mid-end nor the back-end has any
+use for at all.
+
+A front end might respond to the last value by passing the key on to
+something else that might be interested in it.
 
 The following additional values of \c{button} are permitted to be
 passed to this function by the front end, but are never passed on to