Remove signals that are only used internally

This removes the only dependency of the Private class on the TreeView
class. Now the Private class only depends on QTreeView. This enables
sharing the Private class with TreeWidget to get rid of the code
duplication.

(cherry picked from commit 909ac982f00caed0bb03ffbc9367b4e1e249e168)

• GnuPG-bug-id: T6568