TapHandler.exclusiveSignals: you can only tap once if m_doubleTapTimer is used

It's manually testable now with https://codereview.qt-project.org/c/qt/qtdeclarative/+/464993.

Turn on both checkboxes to set exclusiveSignals: TapHandler.SingleTap | TapHandler.DoubleTap

Now you can either single-click or double-click, and you'll see appropriate feedback from the singleTapped or doubleTapped signal, only once. After that, you won't see the feedback again.