A bunch of improvements and fixes for tree widgets (ObserverWidget in
tree view mode):

[+] ObserverWidget/TreeWidget now has a new refresh mode which makes
refreshes much smoother than previously. The new mode (which is also the
default) overlays a progress indicator on top of the current tree and
smoothly refreshes the tree, instead of hiding the tree and showing a
progress indicator instead of the tree. The old behaviour is still
around to use if you prefer to do so. See ObseverWidget::RefreshMode for
more details.
[#] ObserverWidget will no longer register all its actions by default,
only when ActionHints are specified on the displayed observer and the
ObserverWidget is set to use its observer hints, instead of custom
hints.
[#] A few optimizations and possible crash situations fixed in
ObserverTreeModel and ObserverTreeModelBuilder.
[*] Fixed ObserverWidget not respecting the
ObserverHints::ItemViewColumnFlags of new selections in TreeView mode.
Updates to ObjectManagement, MainWindow and BuildingTreeStructures
examples.
More work on ObserverWidget and ObserverTreeModel:

[+] Added required functions to get expanded objects in an
ObserverWidget tree, and to expand specific objects. Previously this
could have only been done using the names of items, which obviously
caused issues when duplicate names were present in a tree.
[*] Fixed ObserverWidget not keeping its selection when refreshing while
using the ObserverHints::RootIndexHide hint.
[*] Fixed ObserverWidget not keeping its expanded nodes when refreshing
while using the ObserverHints::RootIndexHide hint.
Fixed bad performance in ObserverWidget trees during tree expansion and
collapsing operations experienced when automatic column resizing was
enabled.
Lots of small fixes and improvements to Observer models and
ObserverWidget. This is still work in progress...
A whole bunch of changes to ObserverWidget still. This commit does not
contain 100% working code, but creating a reference point for changes
I'm about to make.
Finally cleaned up ObserverWidget internals related to FollowSelection
activity filter hint. Its much much faster now and also a lot cleaner
and easier to maintain.
ObserverWidget polishing and improvements mostly done I think. More
testing tbd to fix any issues introduced by the changes.