Move UbuntuWebContextMenu / UbuntuWebContextMenuItem to qt/quick/api

The initial iteration of the API to allow the context menus to be customized will live in qt/uitk/lib. There are a couple of reasons for this:

- It makes use of the Action class from Ubuntu.Components
- qt/uitk/lib is experimental, so it doesn't matter if we break that for now.

It will be possible to break the dependency on Action by delegating from qt/quick to qt/uitk/lib via AuxiliaryUIFactory. There isn't a build time dependency on Action anyway because there's no C++ API for it, so we just pass Actions around as QObject pointers.

It turns out there's a fair amount of code involved with UbuntuWebContextMenu / UbuntuWebContextMenuItem, so it would be nice for this to move to qt/quick instead and not have it tied to the UITK implementation.