Gnats 6057: Support adding user slots to Element Objs; [1] Preparation
Bug Number: 6057
Release notes (y/n): Yes
For Release Nums: 7.2

This commit is preparation for supporting directly adding User Slots to 
Element Objects (on Aggregate Objects).

Each of the fifteen (15) "Add Slot" operations were directly calling 
SimObj::createUserSlotWithDefaultName() on the _simObj field.  These now 
all call the following new local method which can determine of it's 
appropriate to instead create the new slot on the selected Element Object.

   Slot* OpenObjectDlg::addSingleNewUserSlot (
      const QString& baseName, 
      const QString& slotType,
      SimObj* targObj = NULL);

This commit also includes the following enhancement.

   When a new slot is added, the newly added slot is scrolled to 
   in the Slots list) and selected (as the only selected slot item).

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