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.