SCT: Slot Description Tooltips on slot items in the slot-list tabs
Bug Number: n/a
Release notes (y/n): Yes
For Release Nums: 6.5

As part of a BOR Truckee task to Show Slot Descriptions in the SCT, a slot's 
description is now shown as a tool tip in the following thee places.  
(See also the two prior related commits).

 (1) SCT Edit Series Slot List Tab
 (2) SCT Scalar Slots Tab (including 1x1 Table Slots)
 (3) SCT "Other Slots" Tab (Table, Periodic and List Slots)

When a slot description is shown as a tooltip, the original line-breaks are 
preserved AND lines longer than 90 characters are split into multiple lines 
-- i.e. long lines are wrapped.

Note: The latter two use our general SlotListPanel class.  This is also used 
in the "Find Inputs / Descriptions" dialog, so this tooltip enhancement 
applies to that dialog as well.

ALSO: a "Step Size" column was added to the SCT Edit Series Slot tab.  
This is especially useful because the SCT series data table can support 
only one step size at any one time.  Series slots which don't match that 
step size are grayed-out, but there is not a good indication of why.  
(Another reason could be that the slot item -- specified using a complete 
slot name -- doesn't actually exist in the model).

A new callback was added for notifications to changes to a Slot's description.  This is issued on slots and SimObjs.
   SLOT_DESCRIPTION_CHANGED
   ... with callback data class: SlotStatusChangeData

Maintenance: Use of the deprecated QString::null object was replaced with 
default-constructed QString -- and similar treatment for literal empty 
strings used to initialize QStrings.  Also, application of the new "override" 
feature on virtual method overrides.

See the project analysis and proposal:
 http://cadswes2.colorado.edu/~philw/2014/BorTruckee/ShowSlotDesc/proposal.html

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