Use the new context menu API

[Croydon]

With browser.menus.overrideContext() it is now possible again to use native context menus with unlimited custom entries + other add-ons can display their entries there as well. Finally!

This issue is about migration the custom context menu to the new extension APIs.

[grahamperrin]

Hi, are there any show-stoppers to this?

[Croydon]

No, I just didn't come around to work on VTR in a long time, but hopefully I can make time again in the next weeks 😄