QmlProfiler: Handle enable and show signals for zoomslider in C++

As it's hard to pass arguments for signals from non-QML-mapped objects
to QML objects handle the signal in C++ instead and just directly set
the properties.

Task-number: QTCREATORBUG-10943
Change-Id: I039f6938db3d7e64ca1a4bcff2f0f6aa79c65219
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>