Support multiple pages in external help viewer

Based on QTCREATORBUG-20554 it is possible to add new "Open Pages" to the Qt Help view. In fact, when the user right clicks on a link (in the Creator main window help view) he gets multiple options one being "Open Link as New Page". Unfortunately this is not possible when creator is configured to use an external help window. In such an external viewer the number of available context menu options is very limited and the "New Page" option being absent entirely.

The context menu on right click on link in the external help viewer should match the options of the help viewer in the main Creator help window. The user can only assume that the middle mouse button has the same affect.