Make PageView::notifyViewportChanged call "itself" with a queued connection

This increases the UI responsiveness on opening PDF that are slow to render
This is because we can only have one thread using the pdf because of
poppler, if we do not queue this call, it will immediately try to generate
the pixmap for this page ending up in the thread for rendering being started.
But then while we are still opening the file we will ask if the file
has embedded files, and this means accessing poppler again and we will be
stuck there because the rendering thread has still not finished. This way by
delaying the execution until we return to the event loop we guarantee
all short queries that are done while opening the file are done already and
we won't block the UI