Fixed: (1) Prevent renaming of predefined link groups; (2) Insure uniqueness of user-defined display groups
Bug Number: 5170, related issues. Ready to close.
Release notes (y/n): Yes
For Release Nums: 6.3

Gnats 5170: Allowed to delete (and rename) the default display group property corrupting the model file.
https://cadswes2.colorado.edu/internal/cgi-bin/gnats-query-single.pl?Number=5170

Prior code changes had prevented the deletion of, or direct editing of the name of either of the predefined link groups ('Main Channel Links' and 'Default') within the Display Group Editor (as inline editing in the treeview). But it was still possible to change these predefined link group names in their editor dialog. Now, the group name QLineEdit in those editor dialogs is made read-only when the Link Group being edited is a predefined group. (Currently, this applies only to LINK groups).

In both places where group names can be changed, it had been possible to rename a user-defined link group TO 'Main Channel Links' and 'Default' -- or to any other already existing link group.  In fact, group-name uniqueness was not guaranteed for the other display group types either: Object, Account and Supply Groups. Now, group name uniqueness (with respect to each of the four display group types) is now enforced.

Some internal issues in these classes were also addressed.
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