avcodec: use a choice list for --avcodec-skip-idct as per description

Gives much better UX in gui prefs.

Reviewers, please carefully check that the text is correct. It's a long time since I made this patch; I vaguely recollect that the labels are a merge of those from frame_skip_list_text and nloopf_list_text with the "B-frames" form from the original option label. If correct, perhaps the same new labels would be better for those two lists also? Or would it be better to use the shorter form of the existing lists?