New puzzle: `Untangle', cloned (with the addition of random grid

generation) from a simple but rather fun Flash game I saw this
morning.

Small infrastructure change for this puzzle: while most game
backends find the midend's assumption that Solve moves are never
animated to be a convenience absolving them of having to handle the
special case themselves, this one actually needs Solve to be
animated. Rather than break that convenience for the other puzzles,
I've introduced a flag bit (which I've shoved in mouse_priorities
for the moment, shamefully without changing its name).

[originally from svn r6097]

list.c

@@ -32,6 +32,7 @@ extern const game samegame;
 extern const game sixteen;
 extern const game solo;
 extern const game twiddle;
+extern const game untangle;
 
 const game *gamelist[] = {
     &cube,
@@ -49,6 +50,7 @@ const game *gamelist[] = {
     &sixteen,
     &solo,
     &twiddle,
+    &untangle,
 };
 
 const int gamecount = lenof(gamelist);