Context menu popups with copy, cut, and paste items

Input fields get a default context popup menu with just Copy, Cut, and Paste options. Using install_contextpopup_callback() on an input field replaces the default popup menu with a custom one. copycutpastemenu.C gives an example of adding the default options to a custom popup menu. copycutpastemenu.C creates a context popup menu for an input field containing just these default options, so the end result looks just like the default popup, but is trivially extendible with any other options.

      const x::w::input_field &ifield,
      const x::w::callback_trigger_t &trigger,
      const x::w::busy &mcguffin)
          // ...

          x::w::container context_popup=
               (const x::w::listlayoutmanager &llm)
                   // ...

                   ifield->append_copy_cut_paste(IN_THREAD, ifield)

copycutpastemenu.C gives an example of following the rules for managing references to the popup menu and its callbacks, and correctly creating and destroying the context popup menu after it's no longer needed.