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/tents.sav

@@ -0,0 +1,32 @@
+SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
+VERSION :1:1
+GAME    :5:Tents
+PARAMS  :5:8x8de
+CPARAMS :5:8x8de
+DESC    :45:ea_ddidfaabkd,3,0,2,1,2,2,1,1,3,1,1,1,1,1,3,1
+NSTATES :2:25
+STATEPOS:2:25
+MOVE    :9:N5,6;N6,6
+MOVE    :14:N5,7;N6,7;N7,7
+MOVE    :9:N0,7;N1,7
+MOVE    :4:N1,6
+MOVE    :14:N6,2;N6,3;N6,4
+MOVE    :9:N7,2;N7,3
+MOVE    :14:N1,0;N2,0;N3,0
+MOVE    :4:N3,1
+MOVE    :4:N0,3
+MOVE    :4:N3,4
+MOVE    :4:N5,4
+MOVE    :4:T6,0
+MOVE    :4:T4,0
+MOVE    :4:T0,0
+MOVE    :4:N1,1
+MOVE    :4:N4,1
+MOVE    :9:N6,1;N7,1
+MOVE    :4:T2,1
+MOVE    :9:N1,2;N3,2
+MOVE    :4:T5,2
+MOVE    :4:N4,2
+MOVE    :4:N5,3
+MOVE    :4:N0,2
+MOVE    :9:N1,3;N1,5