Fix insertion of text characters in math

Fix insertion of text characters in math

Characters that needed to be inserted in text mode in mathed were not
correctly inserted. Here we do two fixes:

1/ in niceInsert(), do not replace the contents of the active insets
first cell with selection, insert selection instead. This wa sthe
cause of the bug: an empty selection replaced the contents that was
already in the cell.

2/ do not use niceInsert() anyway, insert() is perfect for what we
want to do.

Fixes bug #11527.