folderview: port the context menu away from KonqPopupMenu.

Many actions were coming from this code anyway, all that was missing
was:

• Copy To / Move To (provided by KFileCopyToMenu)
• the properties dialog (7 lines for action+slot/lambda)
• popup menu plugins, now provided by KIO 5.27's new method in KFileItemActions

So this commit loses plugins with KIO < 5.27, but that's probably
not a big deal.

Differential Revision: https://phabricator.kde.org/D2914