[Feature Request] Add a mode for "speedbar".

[shamefulCake1]

May I ask that mu4e-overview supply a "speedbar minor mode" wrapping it?

Seemingly the data model would map very well to the tree widged "supported" by speedbar, and although speedbar is not the best example of code, it is shipped with Emacs, and can be exploited.

There already is a prototype of the code in the stock mu4e, called mu4e-speedbar, but it is also not a "speedbar mode".

https://github.com/djcb/mu/blob/master/mu4e/mu4e-speedbar.el