        Use OS_CheckModeValid to disallow Wimp_SetMode to invalid mode
        Stop menus from being forced back on-screen on right-click
        Keep submenus above iconbar as well
        Move parent menu if submenu is at edge of screen
        allow panes in menu dboxes
        re-open menu tree beyond dboxes if required
        optimise panes entirely within tool window
        fix bug re: pointer palette being reset to Wimp palette
        Restore command / error window width after expansion
      * make rectangle area expand dynamically
      * Wimp_LoadTemplate shouldn't load indirected data to scratch buffer first
        System font versions of chars &8F..&9F
        Make instant dragging the default in the Wimp
        Use Wimp_CommandWindow on Shutdown.
        Restore caret correctly on menu re-open.
        Return pointer to scroll bar after a 2D drag
        Wimp_GetMenuState can't tell you if the menu tree is open - if the
           pointer is not on the root menu and there are no submenus open.
