A new enum UnknownUrlSchemePolicy was added to WebEngineSettings,
QWebEngineSettings and QQuickWebEngineSettings.
WebContentsDelegate now has a new attribute of that type, which can
be read and written through the public APIs
Q(Quick)WebEngineSettings::unknownUrlSchemeNavigationPolicy and
Q(Quick)WebEngineSettings::setUnknownUrlSchemeNavigationPolicy.

This way, one can control, whether URLs with unknown schemes are passed
to QDesktopServices. WebContentsAdapterClient::navigationRequested is
called on these requests, so this allows more fine-grained control
over the schemes and origins, that are allowed.

Task-number: QTBUG-58627
Change-Id: Ie81d9503456d63ea1ed5606483254acf437cd8f7
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>