QmlDesigner: fix using the right object/unit

TreeViewStyle is existing two times and wasn't in anonymous namespace.
The result after refactoring was that the wrong TreeViewStyle was used.
Moved the two times existing drawSelectionBackground to NavigatorTreeView
to avoid similar problems.

Change-Id: Id87894a01b5f6c94c81c478f9492ec6b78eb9e70
Reviewed-by: Marco Bubke <marco.bubke@digia.com>