Lack of delete button in INTKEY

[GoogleCodeExporter]

Reported by Dr. Hans Georg Richter (Jorgo)

ISSUE04: A USED character is automatically displayed in the respective window 
"used characters". In many cases (for instance with all dichotomous characters) 
it is not possible to "undo" the action and return the USED CHARACTER to the 
AVAILABLE CHARACTERS window. One can change the selected character state but 
cannot deselect both and thus eliminate the chosen character from the USED ones.

This functionality can be achieved in advanced mode using the DELETE directive, 
however I can see the value in providing a toolbar button to allow a character 
value to be deleted.

Original issue reported on code.google.com by [email protected] on 11 Nov 2013 at 3:47