Change license from MIT to MPL 2.0.

Also add emacs and vim modelines.
Update for the license change.
Scripts to change licenses and add modelines in source files.
Support a way to append strings without checking for duplicates.

This is needed in case of duplicate strings one of which is unformatted
while the other has format runs. In xlsx these two are assigned different
string ID's.
Surround literal strings with double-quotes when printing formulas.