Introduce qml APIs to support custom dialogs:
 * http and proxy authentication
 * javascript dialogs
 * file pickers
 * color pickers
 * form validation messages

[ChangeLog][QtWebEngine][QML] Added ability to provide custom dialogs
for HTTP and proxy authentication, JavaScript alerts, file and color
picking, and form validation messages.

Task-number: QTBUG-51190
Change-Id: I559ca59264750feb36ba7d3dba0bf0647509306d
Reviewed-by: Kai Koehne <kai.koehne@qt.io>