Update QWebEnginePage::WebWindowType

Expose the same values as currently available in
QQuickWebEngineView::NewViewDestination.

Rename the WebModalDialog to WebDialog, which actually replaces the
tool/status/menuBarVisibilityChangeRequested signals.

Change-Id: Icc103f434fb3eca49f1a53e476e101c3d6fffd36
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>