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:

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

[enricozb]

Thanks for writing these out, I've checked some of them off as they have been added in 5842d44. I'll close the issue once I make a decision about all of these.