fdo#72691: Write unit test for this.

One tricky bit was that in order to reproduce the bug in the unit test,
formula cell's bChanged flag needed to be cleared after each update,
because that's what happens when you display updated formula on screen.  Each
time an updated formula cell gets drawn, the UI code clears this flag.  That
same flag was used to control the code path during interpretation...

Change-Id: I2eedea3c9294f4f545422b8611840c81f8c2304f
fdo#72691: Allow overwriting of string value with numeric one.

This can legitimately happen when you have a matrix with a reference to
another cell inside, and the referenced cell originally contained a
string value then later overwritten by a numeric value.

Example.  Put a "Text" in A1, and in B1 put a 1x1 matrix {=A1}.  It
displays "Text" in B1.  Then put 11 in A1.  Prior to this change, B1
would become blank.  With this change, B1 will display 11.

Change-Id: I3feba3a8658e1a5ebf6f9e5ac34de2d579464ddb