#2 Provide a way to remove possible numbers

[Nick Crossley]

When you double-click a cell and it turns yellow, you
can add a list of possible numbers. This patch provides
a way to remove numbers from that list. When the
patch is applied, if you just type a digit in a yellow
square, the digit is added to the list if it is not already
there, just as before. However, if you hold down the Alt
or Meta key while the digit is typed, the digit will be
removed from the list if it wass previously there.

Additionally, this patch changes the list to use a
smaller font to represent lists of more than 4 possible
digits, instead of just throwing away all but the four
most recently entered digits.