Use QSet instead of QHash for string pooling

Apparently the iterator returned by insert() is not used anymore.
Use objectListModel() instead of the tree model
Drop SignalHistoryModel::Item::updateFromModel()

Right now the signal monitor doesn't track object name changes (simply because
neither ObjectTreeModel or ObjectListModel report such events). Therefore this
method is not needed.
Simplify the internString() function

With SignalHistoryModel::Item::updateFromModel() gone we don't need the
identity check anymore. It's gone anyway.