drop support for QtWebKit. (#680)

drop support for QtWebKit. (#680)

Our QtWebKit build didn't work any longer on Ubuntu bionic or focal,
but went into an infinite loop if map preview was used.
I observed this behavior with builds of 1.7.0 as well.

Document exisiting gui configuration option disable-mappreview.

I note that latest fedora build spec falls back to QtWebKit on some
architectures where QtWebEngine is not available.  This change will
require them to fall back to disabling the map preview instead.
https://src.fedoraproject.org/rpms/gpsbabel/blob/rawhide/f/gpsbabel.spec

QtWebEngine appears to be coming to Qt6 with or after 6.2.
https://bugreports.qt.io/browse/QTBUG-63235