New mechanism for automatic generation of the puzzle screenshots on

the web, which I hope will also end up being extended to generate
both Windows and X icons for each individual puzzle. The mechanism
is: for each puzzle there's a save file in the `icons' subdirectory
showing a game state which I think is a decent illustration of the
puzzle, and then there's a nasty set of scripts which runs each
puzzle binary, loads that save file, grabs a screenshot using xwd,
and munges it into shape.

In order to support this I've added two new options (--redo and
--windowid) to all the GTK puzzles, which I don't expect ever to be
used outside the icons makefile. I've also added two more options
(--load and --id) which force a GTK puzzle to treat its command-line
option as a save file or as a game ID respectively (the previous
behaviour was always to guess, and sometimes it guessed wrong).

[originally from svn r7014]

icons/sixteen.sav

@@ -0,0 +1,39 @@
+SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
+VERSION :1:1
+GAME    :7:Sixteen
+PARAMS  :3:4x4
+CPARAMS :3:4x4
+SEED    :15:601798566229573
+DESC    :38:2,16,3,10,13,8,7,4,9,14,12,11,15,1,5,6
+NSTATES :2:31
+STATEPOS:2:24
+MOVE    :5:C3,-1
+MOVE    :4:R0,1
+MOVE    :4:C1,1
+MOVE    :5:R0,-1
+MOVE    :5:C1,-1
+MOVE    :5:C3,-1
+MOVE    :4:R2,1
+MOVE    :4:R2,1
+MOVE    :4:C3,1
+MOVE    :5:C2,-1
+MOVE    :5:C2,-1
+MOVE    :4:R1,1
+MOVE    :4:R1,1
+MOVE    :4:C2,1
+MOVE    :4:C2,1
+MOVE    :4:R3,1
+MOVE    :4:R3,1
+MOVE    :4:C1,1
+MOVE    :5:R2,-1
+MOVE    :5:R2,-1
+MOVE    :5:C1,-1
+MOVE    :4:R2,1
+MOVE    :4:C0,1
+MOVE    :4:R3,1
+MOVE    :5:R2,-1
+MOVE    :5:C1,-1
+MOVE    :4:R2,1
+MOVE    :4:C1,1
+MOVE    :5:R3,-1
+MOVE    :5:C0,-1