Start keybinding combination

[Ergus <spacibba@aol.com>]

Hi:

I have been wondering if there is a hook or advise I could use to give
some more explicit visual feedback when the user starts a long
keybinding.

I mean:

If a user wants to start a command like `C-x r b` I would like to change
the mode-line color after the `C-x`. I suppose that this may be a call
to a hook in the same place that updates the echo area inserting the
current prefix.

I saw that packages like which-key needs to do a pooling timer to
emulate that, so maybe this needs a feature request or is there a reason
to not implement this?

Best,
Ergus