Add the ability to use the Rectangles solver for actually solving

puzzles, rather than just doing its nondeterministic number
placement thing. This enables the use of the `Solve' menu option on
externally entered game IDs, provided of course that they aren't
_too_ difficult.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5852 cda61777-01e9-0310-a592-d414129be87e