Optimized the way ObserverWidget handles collapsing and expanding of
nodes when in tree mode. It no longer re-calculates all expanded/
was expanded/collapsed.

During benchmarking, this change reduced the time a simple node
expansion/collapsing operation on a tree with 10000 items takes from 26
seconds to less than 1 second.
No longer post process event updates during rebuilding of internal tree
structure in ObserverTreeModelBuilder. This causes the display to
flicker while rebuilding the tree in ObserverWidget.