Generalise the deletion protection mechanism from math to text (#9540)

Generalise the deletion protection mechanism from math to text (#9540)

Now backspace and delete in text will select non-empty math and text insets
before deleting them. This is consistent with what happens in math already.

This is implemented for InsetText as well but can be disabled in case of
negative feedback.

This can be set for any sort of inset with the new virtual method
Inset::confirmDeletion.

New option "force" for the LFUN_*_DELETE_* commands, that bypasses the
confirmDeletion check.