Unify API of TreeView and TreeWidget and reduce code duplication
Use TreeViewPrivate as (base class for the) private class of TreeWidget
to get rid of most duplicated code. Moreover, we get the two column
menus for free so that they can now also be used for the TreeWidgets in
Kleopatra.
(cherry picked from commit cf6939f966aeacc4e32e229daa3db9f39bc7b07a)
- GnuPG-bug-id: T6568