Made menu bar's autoloaded stuff conditional on

Made menu bar's autoloaded stuff conditional on
purify-flag, to avoid error message when loading ediff in a frame
that has no menu bar.

(ediff-setup): Improved mode-line-buffer-identification.
Now accommodates buffer identifications generated by
mode-line.el and uniquify.el.

(ediff-current-diff-face-A/B,
ediff-fine-diff-face-A/B,ediff-odd/even-diff-face-A/B):
Variables changed to contain face names
instead of face internal representation.