Implement Popup menu support.

Current implementation uses EFL as it's starting point. Files were
moved to shared locations and some WK API files were added. They allow
POPUP_MENUS implementation to be shared between different platforms.

WKPopupItemPlatform uses this name as it will be shared by Nix, EFL and
Qt. When moving to upstream it should be renamed to WKPopupMenuItem.
Another rename should be made for WKPopupItem -> WKPopupItemMenuData
(same model that is used by CONTEXT_MENUS).

Reviewed by: Rafael Brandao
             Jesus Sanchez-Palencia