Description of Problem:
When using a script to generate a signature, once a script is entered it
appears to be impossible to change which script is used.  One must instead
remove the signature definition and create a new one.

Steps to reproduce the problem:
1. Create a script-generated signature with path X.
2. Verify signature works by looking at preview.
3. Edit signature and change path to a different script; save.
4. Notice that the preview doesn't change.
5. Edit signature again and notice that the path is not changed.

Actual Results:
Script paths are not editable, even though they appear to be so.

Expected Results:
Uh, I expect to be able to edit the path.

Additional Information:
Combining this bug with #34397 is really annoying -- I wrote a script that
works with multiple mailers (Mutt, Pine, Thunderbird, etc.) for multiple
accounts, and I use arguments to specify how to format everything and what
content to use.  I try to use this script for Evolution and it fails
because Evolution cannot execute scripts with arguments.  So I try to edit
the script to point to a link, after modifying the script to support
divining arguments via script name, and it still doesn't work.  Ugh.  I had
to recreate all of my signature definitions.