[FIX] mass_mailing: properly manage td/th elements

Refactore width-x snippet option and create another option to handle
cloning and removing td elements properly.

* on_focus: the handle to resize are now hidden when the td/th is alone
on its row

* on_clone: when cloning a td element which is alone on its row, it is
instead cloned in a new row. If not, empty td are added on the whole
column to maintain table integrity.

* on_remove: handle removing td in complex table