Now that we're highlighting the currently selected preset in the

Now that we're highlighting the currently selected preset in the
Type menu, it looks faintly silly that Fifteen doesn't have any
presets other than Custom: you open a Fifteen window in its default
state, and the Type menu appears to be telling you it has a custom
size! Fixed by adding a preset for the default parameters.

I'd quite like to fix this properly by revamping the presets
mechanism in a way that _enforces_ that there must always be a
preset which matches the default parameters, but that's more fiddly
than it sounds. For the moment, this change fixes the only
externally visible infelicity in the current game set.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@7983 cda61777-01e9-0310-a592-d414129be87e