Attribution control on Qt

[1ec5]

QQuickMapboxGL::setCopyrightsVisible() is unimplemented. The Qt SDK should automatically provide an attribution control that displays copyright statements and links to further information.

/cc @tmpsantos

[tmpsantos]

Depends on #5999

[1ec5]

This work was actually unblocked by #6431.

[brunoabinader]

As part of the shift towards the QGeoMap plugin for Mapbox GL as a replacement for QQuickMapboxGL, we provide a signal copyrightsChanged(QString) in #7347.