Move popup menu implementation to Nix specific files and rename WKPageSetPagePopupMenuClient to WKPageSetUIPopupMenuClient.

The implementation can't be on main WK2 files, despite of being almost the same as
the Efl implementation, it's not used by other WK2 ports.

To keep Nix in sync with Efl the function WKPageSetPagePopupMenuClient was
renamed to WKPageSetUIPopupMenuClient.

Reviewed by: Rafael Brandão