Unruly: new 'Trivial' difficulty level.

A user recently mentioned having found even 'Easy' to be harder than
they'd like. That difficulty level generates puzzles that can be
solved by filling in any square for which the other colour would
immediately generate a run of 3, and spotting rows that have the
correct total number of one colour and filling in all remaining
squares in the other colour.

Initially I thought there wasn't much I could do to make the solution
techniques easier than that. But after a bit of thought, I decided the
second criterion can be weakened a bit. The new 'Trivial' level
replaces it with a special case: when a row or column only has _one_
remaining unfilled square, the remaining square is filled in by
counting.

That version of the rule doesn't require the player to do any counting
in order to _spot_ possible applications of it: you can see at a
glance that a row or column has only one remaining grey square, even
if having seen it you then have to count to work out which colour to
fill it in. So it makes a gentler introduction to the game.