Register custom protocols using QWebEngineUrlScheme

Summary:
From Qt 5.12.0, before installing custom url scheme handlers, the scheme
itself must be registered using QWebEngineUrlScheme.

Registering URL schemes must be done before instantiating any
QWebEngine class, so we do that at the beginning of WebEnginePart's
constructor using a static function which only does the registration
the first time it's called.

Having to register url schemes before creating QWebEngine classes
creates a problem beacuse we install scheme handlers dynamically from
WebEnginePart::openUrl if the default mimetype for the URL (according to
KProtocolManager::defaultMimeType) is 'text/html'. To avoid this issue,
we use KProtocolInfo::protocols to obtain a list of all availlable
protocols, create a fake URL for that protocol (as there's no way I can
see to obtain the default mimetype for a protocol from KProtocolInfo)
and use KProtocolManager::defaultMimeType on this URL to decide whether
to register the URL scheme or not.

Using QWebEngineUrlScheme also allows to specify that a given protocol can
access local files, removing the need to use the html embedder from
WebEnginePartKioHandler. Since this only works with Qt 5.12 or later, the html
embedder is only disabled depending on the Qt version in use.

Test Plan:
run Konqueror with Qt 5.12 and check that no warnings about
registering custom protocol are displayed on the terminal. Ensure that protocols
such as about:, error:, man:, info: work correctly (for example, try man:cp or
info:cp)

Reviewers: dfaure

Reviewed By: dfaure

Differential Revision: https://phabricator.kde.org/D21741