Inserted signatures should replace, not add each other

[humdinger]

This is hrev33893.

With ALT+1,2,3... there's a quick way to append a predefined signature to your Mail. If you don't find the right one on first try, however, pressing ALT+1,2,3... will add one sig under the other.

It'd be much nicer to always have a signature replace the one inserted before. In order not having to track what's a signature, this could be as easy as to keep an inserted sig text selected. That way inserting another one will simply replace it.