* Annoyingly although Qt has an internal bool allowUserMoveOfSection0
which does exactly what we want allowing the tree column to be
movable, we can't enable it because it's private. So instead we have
to re-implement section moving ourselves.
* Minor tweak - also made RDTreeWidgets non-movable by default since
usually we don't want to allow it.
* Since we're promoting everything, we reset the behaviour of
RDTreeWidget so that it's not doing anything different by default.
* RDTreeWidget's interface is a bit different, exposing some useful
things like a single selected item and so on.
* We also can't set columns in the Qt Creator UI anymore, so we set them
from code.
* It seems due to deficiencies in Qt's design, there will be a
substantial need for these small custom subclasses just to add basic
functionality that should have been present in the original. So we'll
adopt a naming scheme of QFooBar -> RDFooBar for this 'almost the
same widget' type subclassing, compared to more custom/changed ones.
* GUIInvoke helper changed to use QMetaObject::invokeMethod which works
on threads better.
* LambdaThread helper class now has a thread member, it doesn't derive
from thread (this seems to be recommended practice).
baldurk