Add plugins.qmltypes for QtWebView import

This should fix warning from Qt Creator and auto completion.

Task-number: QTBUG-42665
Change-Id: I16c6856e699ebe522535e534eb5a5c015db69409
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Caroline Chao <caroline.chao@theqtcompany.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@theqtcompany.com>