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/net.sav

@@ -0,0 +1,53 @@
+SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
+VERSION :1:1
+GAME    :3:Net
+PARAMS  :3:5x5
+CPARAMS :3:5x5
+DESC    :25:1115337157375775157135131
+UI      :9:O0,0;C2,2
+NSTATES :2:45
+STATEPOS:2:45
+MOVE    :4:C0,0
+MOVE    :4:L0,0
+MOVE    :4:L0,1
+MOVE    :4:C0,2
+MOVE    :4:L0,2
+MOVE    :4:A0,3
+MOVE    :4:L0,3
+MOVE    :4:L0,4
+MOVE    :4:L1,4
+MOVE    :4:A2,4
+MOVE    :4:A2,4
+MOVE    :4:L2,4
+MOVE    :4:C1,0
+MOVE    :4:L1,0
+MOVE    :4:L3,0
+MOVE    :4:L2,0
+MOVE    :4:A4,0
+MOVE    :4:A4,0
+MOVE    :4:L4,0
+MOVE    :4:A4,1
+MOVE    :4:L4,1
+MOVE    :4:L3,1
+MOVE    :4:A3,2
+MOVE    :4:A3,2
+MOVE    :4:L3,2
+MOVE    :4:L2,2
+MOVE    :4:A2,1
+MOVE    :4:A2,1
+MOVE    :4:A1,1
+MOVE    :4:A1,1
+MOVE    :4:A1,1
+MOVE    :4:A1,1
+MOVE    :4:A1,1
+MOVE    :4:A1,1
+MOVE    :4:A1,2
+MOVE    :4:A1,2
+MOVE    :4:A1,3
+MOVE    :4:C2,3
+MOVE    :4:A3,3
+MOVE    :4:A4,4
+MOVE    :4:A4,4
+MOVE    :4:A4,3
+MOVE    :4:A4,2
+MOVE    :4:A4,2