Support preferences in the GTK frontend.

Finally, some user-visible behaviour changes as a payoff for all that
preparation work! In this commit, the GTK puzzles get a 'Preferences'
option in the menu, which presents a dialog box to configure the
preference settings.

On closing that dialog box, the puzzle preferences are enacted
immediately, and also saved to a configuration file where the next run
of the same puzzle will reload them.

The default file location is ~/.config/sgt-puzzles/<puzzlename>.conf,
although you can override the .config dir via $XDG_CONFIG_HOME or
override the Puzzles-specific subdir with $SGT_PUZZLES_DIR.

This is the first commit that actually exposes all the new preferences
work to the user, and therefore, I've also added documentation of all
the current preference options.

puzzles.h

@@ -388,6 +388,8 @@ void free_cfg(config_item *cfg);
 void free_keys(key_label *keys, int nkeys);
 void obfuscate_bitmap(unsigned char *bmp, int bits, bool decode);
 char *fgetline(FILE *fp);
+char *make_prefs_path(const char *dir, const char *sep,
+                      const game *game, const char *suffix);
 
 /* allocates output each time. len is always in bytes of binary data.
  * May assert (or just go wrong) if lengths are unchecked. */