qt/qml: use theme aware attached tooltip

There were previous attempts to allow using customized tooltip as attached tooltip.

!661 (closed), which was too verbose. And !693 (closed) was invasive that a custom style had to be used (this seems to be the default way, but it also had other problems)

When I looked into the Qt source code, I realized that an internal property within the QML engine is used to access the global tooltip instance. With this merge request, I added a new function to set this internal property to a custom QQuickToolTip so that any Qt Quick Tool Tip derived object can be set as the attached tool tip.