Move more back-endish menu-related stuff from xboard.c to menus.c

Move more back-endish menu-related stuff from xboard.c to menus.c

Move menu enabling to menus.c
 Only the wrapper EnableMenuItem stays in xboard.c. A new wrapper
 EnableButtonBar was necessary, because Training mode wants to enable/disable
 that too.
Move all back-endish MenuProcs to menus.c
 Many if the MenuProcs that could not be eliminated by listing the
 corresponding back-end MenuEvent function as handler directly, are
 still completely plactform indpendent. (E.g. when the MenuEvent needs
 a parameter.) These are all moved to menus.c now.
Convert all dead MenuProcs to using MarkMenuItem
 The MenuProcs for the option items that no longer exist were also
 converted for check-marking their item in the new way, because we
 might revive them for key binding.
Move the dead MenuProcs also to menus.c
Revive dead menu procs
Move GreyRevert to menus.c
Move ModeToWidgetName to menus.c
Move initial menu marking to menus.c

A new function InitMenuMarkers was split off from main() to do this.