Adds a recent list to the frame multiplexer + small enhancements.

[mychris]

Some enhancements for the frame-multiplexer.

• Adds a recent list, so one can jump between recently visited frames. Mostly used to the jump to the most recent.
• Adds names to the frames. If no name is given, the behaviour is unchanged. Names can be removed by giving a frame an empty name

New commands:

• frame-multiplexer-switch jump to a frame with the given ID (prefix argument)
• frame-multiplexer-recent go to a frame in the recent list
• frame-multiplexer-rename give a name to a frame
• frame-multiplexer-nomalize-ids re-assign IDs in case many frames have been deleted and created, so it is easier to jump with frame-multiplexer-switch

Changed commands:

• frame-multiplexer-next Accepts a prefix argument with which the jump can get bigger
• frame-multiplexer-prev Same as next
• frame-multiplexer-delete Accepts a prefix argument with which a specific frame can be deleted

[mychris]

Maybe the frame-multiplexer is a good place to start with feature keymaps, as described in #1208?

I am not sure if such a change would be too disruptive for users, but I would propose to create a new exported variable *keymap*, which is a keymap, assigning all the commands within the frame-multiplexer to this keymap and then assigning C-z to the *keymap*.

[vindarel]

I think the current style is to put all the define-keys at the top of the file, though there is no official rule and I don't know the maintainer's opinion.

[cxxxr]

Indeed, I think that is better.

[mychris]

Fixed, moved all define-keys to the top of the file.

[cxxxr]

I think it's very nice!
Can I merge this PR?

[mychris]

Nothing to add from my side. What do you think about my first comment, should I just create a PR so we can look at it there?

[cxxxr]

What do you think about my first comment, should I just create a PR so we can look at it there?

I agree that users should be able to set a prefix other than C-z.
Looking forward to the PR.