More complete bar menu as in buffers.kak

[sgraf812]

Compared to https://github.com/Delapouite/kakoune-buffers, I'm missing quite a few options in the menu. Here they are:

• [x] a: alternate
• [x] f: find
• [x] D: Delete all
• [ ] h: first
• [ ] l: last
• [ ] m: modified
• [x] o: only
• [x] c: config
• [x] s: scratch

[enricozb]

Thanks for writing these out, I've checked some of them off as they have been added in 5842d447c0a056e31602668d779088cca818f29c. I'll close the issue once I make a decision about all of these.