Gnats 6057: Support adding user slots to Element Objects
Bug Number: 6057
Release notes (y/n): Yes
For Release Nums: 7.2

The Object Viewer and Open Object Dialogs for Adding custom slots are now 
available for Aggregate Object Element Objects.  This includes additions to 
the Element Object context menu (on element object treeview items).  The 
following changes to that context menu have been made:

  (1) "Append" changed to "Append Element"
  (2) Added: "Add Slots" submenu with fifteen (15) items.
  (3) Added: "Paste Slots" (enabled only if there are slot references in 
        the slot clipboard).

Additionally, the following usability enhancements have been implemented:

  (1) When adding a new slot, the Slot list is scrolled to the added slot, 
      and that slot becomes the only selected item.

  (2) When adding a new Element object, the Slot and Method lists are 
      scrolled to that new Element object, and that object becomes the 
      only selected item.

These enhancements are further described in this informal document:
http://cadswes2.colorado.edu/~philw/2017/bugs/6057/changes1.html

Note: An additional dysfunction was discovered in the course of this 
development. This bug will be handled separately: Gnats 6062: Can't delete 
custom slots from elements of an aggregate object, nor move them to a 
different object.

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