Fix QGV scene selection in in-process mode.
Cleanup parentObject(), QQ2 specific handling is now in its own tool.
Unify the implementation of the object list and tree models.

Just like the tree model, we now use the infrastructure for multi-thread
support from Probe, rather than rolling our own. This also means we are
now back to a recursive mutex, which fixes a deadlock when requesting
the decoration role on the list model (due to an internal temporary QFile
during icon loading).
Document objectCreated/objectDestroyed.

There's a few non-obvious implications in multi-threaded applications to
consider here.
Remove obsolete workaround that causes unnecessary warnings nowadays.
Ensure all ancestors are known, not just the immediate parent.

This fixes a crash when showing tooltips in Qt5 applications.