Suggestion: convert dispatchers to use transient

[Hugo Heagren <hugo@heagren.com>]

Org uses various dispatchers, where invoking a command gives the user a
choice of different sub-commands, chosen by pressing a relevant key,
from a list displayed on the screen. Some of these dispatchers include
options which can affect the command chosen. Examples include org-
capture, org-beamer-select-environment and org-export-dispatch.

These desptachers are idiosyncratic, written for purpose, and each
behave differently. They have varying levels of customisability, and
this is reached in different ways for each. Overall, I think the user-
experience could be more consistent and more easily customisable.

Luckily, recent versions of emacs ship with transient.el, a powerful
way of building such interfaces in a consistent and easily extensible
way.

So, I propose to rewrite the current dispatchers as transients. What
does the community think? I would be happy to work on this unless
others strongly object, but I don't  know everything about org, so if
others could help me with a list of other dispatchers which could also
be converted that would be helpful.

Blue skies, Hugo