More customizable maildir shortcuts

[manufactory]

1. Add a :name attribute to mu4e-maildir-shortcuts to display maildirs with long paths nicely
2. Add support for maildir shortcuts without a :key. Alternatively, allow :key to be a custom string.
   I find a custom string useful for hinting at hydras.
   This implements [Support maildir shortcuts without keys #2110], which has conflicts with the current code.

I also added support for separator lines with titles to categorizes maildirs. Something like

* -- Private --
* [jp]  /private/inbox                                         (n/N)
* -- Work --
* [jw]  /work/inbox                                            (n/N)

I didn't include it since it's quite hackish, but I'd be prepared to clean it up and submit it if interested.

Sorry for opening/closing two other pull requests. I'm not too used to the Github workflow :)

[djcb]

Nice work.... I'll get back to this once the 1.9.x work starts. Thank you.

[llmII]

Should wrap in mu4e--main-action-str (since at the least it'll make it look like the rest of mu4e's maildir shortcuts that are specified manually with mu4e-maildir-shortcuts)

[llmII]

There are changes needed for mu4e-ask-maildir (like previously in my prior attempt at a PR #2110). Otherwise you'll get (wrong-type-argument characterp nil) for maildirs where the :key is omitted.

EDIT:
Note, it needs more drastic change if one's using the new keys that are strings actually. Since I'm not I wouldn't know what changes... but I would fathom that a string will not match characterp.

[llmII]

Why so many spaces after \t*? I'm only asking as the space amount seems arbitrary, but perhaps for you it gives you the specific way you want it to look or something (for me it doesn't look as great if I were to take 2 of the spaces out).

[djcb]

Merging this, thanks!

@llmII: it looks good for me, but surely we can tweak the details later.