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/rect.sav

@@ -0,0 +1,17 @@
+SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
+VERSION :1:1
+GAME    :10:Rectangles
+PARAMS  :3:7x7
+CPARAMS :3:7x7
+DESC    :33:a3d2b2a3_2a4a8h2a3c4_2b2c3a3_3c3b
+NSTATES :2:10
+STATEPOS:2:10
+MOVE    :8:R0,6,3,1
+MOVE    :8:R6,4,1,3
+MOVE    :8:R3,6,3,1
+MOVE    :8:R4,4,2,1
+MOVE    :8:R3,5,3,1
+MOVE    :8:R6,1,1,3
+MOVE    :8:R5,0,2,1
+MOVE    :8:R5,1,1,2
+MOVE    :8:R4,3,2,1