Clear up bug #6722 (Linux: menu items cut/copy/paste not working)

To avoid confusion, don't show Cut/Copy/Paste in HTML menus. They'll only work in the native shell, with specially bound native menu items.

Whenever [Linux native menu support](https://trello.com/c/WMB6vtwO/893-linux-native-menus) lands, these menu items will automatically be exposed, and they should work at that point.  In "in-browser" mode, the menu items will forever stay hidden, since there's simply no way to make them work there.