app, libgimp*, plug-ins: replace enum GimpLayerModeEffects by GimpLayerMode
with proper value names. Mark most values as _BROKEN because they use weird alpha compositing that has to die. Move GimpLayerModeEffects to libgimpbase, deprecate it, and set it as compat enum for GimpLayerMode. Add the GimpLayerModeEffects values as compat constants to script-fu and pygimp.