Slightly different implementation of GUI main window Undo.

[Daniel J Sebald]

I've removed another GUI callback to command_editor and put the 
changeset here:

https://savannah.gnu.org/patch/?8534

It's a better approach in several ways from a programming perspective, 
and the end result is an Undo behavior for the main window that is more 
conventional.  If the tab that is in focus does not have some form of 
Undo, the Cntl-Z in the Edit dropdown menu and the green undo icon are 
turned gray.

These two patches are pretty small:

https://savannah.gnu.org/patch/?8532
https://savannah.gnu.org/patch/?8534

Could someone please review these in the next few days.  They factor 
into what I'm working on for next weekend.

Thanks,

Dan