Add View Source API and make the feature available from context menu

[ChangeLog][QtWebEngineQML][QQuickWebEngineView] View Source feature is
now supported
[ChangeLog][QtWebEngineWidgets][QWebEnginePage] View Source feature is
now supported

Change-Id: Icc16da71fc6ec95880897fc9744dd8be8c004e00
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>