Note the purpose of some SPEF constructs, in comments
fix GUI test for recent changes to DEF instance display
replace templated QGraphicsScene derived class with a templated child class and templated constructor, for simplified code
re-implement instances as QGraphicsItem subclasses without grouping, for performance and simplicity
Implement simple instance highlighting in the DEF viewer.  With this, the "instances" command, and the event
loop integration you can have a simple animation running, highlighting instances from the Tcl command line
while the user simultaneously navigates in the GUI window.