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.

misc.c

@@ -3,6 +3,7 @@
  */
 
 #include <assert.h>
+#include <ctype.h>
 #ifdef NO_TGMATH_H
 #  include <math.h>
 #else
@@ -500,4 +501,34 @@ char *button2label(int button)
     return NULL;
 }
 
+char *make_prefs_path(const char *dir, const char *sep,
+                      const game *game, const char *suffix)
+{
+    size_t dirlen = strlen(dir);
+    size_t seplen = strlen(sep);
+    size_t gamelen = strlen(game->name);
+    size_t suffixlen = strlen(suffix);
+    char *path, *p;
+    const char *q;
+
+    path = snewn(dirlen + seplen + gamelen + suffixlen + 1, char);
+    p = path;
+
+    memcpy(p, dir, dirlen);
+    p += dirlen;
+
+    memcpy(p, sep, seplen);
+    p += seplen;
+
+    for (q = game->name; *q; q++)
+        if (*q != ' ')
+            *p++ = tolower((unsigned char)*q);
+
+    memcpy(p, suffix, suffixlen);
+    p += suffixlen;
+
+    *p = '\0';
+    return path;
+}
+
 /* vim: set shiftwidth=4 tabstop=8: */