Pass profile to new web page (that is being created when a `target="blank"`
link is clicked) to avoid running into `Can not adopt content from a
different WebEngineProfile.`.
Otherwise unity builds fail due to macro redefinitions. It doesn't really
matter what name is used in the web view files anyway (as it just changes
some environment variables used for debugging).
* Use colors from palette; with this the custom scrollbars should always
be a plus (and not only when the Breeze style is used)
* React to palette changes at runtime
* The code is disabled under many platforms where the Breeze style is
unlikely to be used anyways.
* The code is disabled if Qt is older than 6.1 because only then the name
of the current style can be determined.
