This moves various stuff around and out of QtWebKit code:
- open_target and hint_target are now in TabData, not on the WebPage
- As much as possible got extracted from WebPage.acceptNavigationRequest
to AbstractTab._on_link_clicked, with a new link_clicked signal added
to WebPage. However, we need to decide whether to handle the request
in this tab or not inside acceptNavigationRequest, so we have some
code duplicated there and in WebEnginePage.acceptNavigationRequest.
- _mousepress_opentarget (i.e., setting the open_target) is now handled
in MouseEventFilter, not in WebView.
3bffb71b55