Do not delete tags on pressing "del" or "backspace" in certain cases

If according to the translate5 configuration it is not allowed to save a segment with deleted tags, then it should not be possible to delete tags via backspace or del keys.

Therefore if you mark a text that includes a tag and hit one of those keys, the tag should not be deleted, but only the text.

Yet it should still be possible to cut and paste the tag.

This is for better translator usability.