Revamp pencil mark placement and sizing. Pencil marks are now sized

Revamp pencil mark placement and sizing. Pencil marks are now sized
and positioned according to how many there are in the cell, rather
than how many distinct digits there are in the entire puzzle. This
means that 4x4 Solo can now be played with pencil marks without
_too_ much difficulty; the marks will still get a bit crowded if you
have more than 12 in the same square, but with luck that shouldn't
happen often, and as long as you're down in the 2-9 range things
should be entirely legible.

[originally from svn r5909]