New feature to allow converting formula cells to static values.

You used to have to convert formula cells to static values by copying
them and pasting them as values onto the original place.  Why not just
allow converting them in place?

This is something I've always wanted to implement.
Write the first test for this formula-to-value feature.

Change-Id: I545328a1c126b3bfeee8c1f662c5eb9a471d33ca
Another test case for this feature.

Change-Id: Ia406baec2867605bf515dac0ba090500e2bbd1ce