Allow using the text properties dialog in mathed

Allow using the text properties dialog in mathed

Until now only the color of the text could be changed by using
the text properties dialog. This commit allows changing all
other properties except for strikethrough. It is possible to
also add underlining with the limitation that the changes
accumulate. This requires other work but I think that underlining
and strikethrough are not so important in mathed and can be
refined at a later time.

Fixes #12958