Document and safeguard usage of custom schemes

The change improves the documentation of QWebEngineUrlSchemeHandler to
notify the requirement of registering a custom scheme before installing
a custom scheme handler.

Also start showing warnings when
QWebEngineProfile::installUrlSchemeHandler() is called before a scheme
is registered. Also show warnings when installing the handler using
QML.

Task-number: QTBUG-72079
Change-Id: If249592ea43fe2f9ad587a6ff4e8c9dedcc5d3d3
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>