[PATCH 4 of 7 path-configs] ui: implement ui.sections()

[Sean Farley]

Gregory Szorc writes:

> # HG changeset patch
> # User Gregory Szorc <gregory.szorc at gmail.com>
> # Date 1423345652 28800
> #      Sat Feb 07 13:47:32 2015 -0800
> # Node ID e93fb90e9b87879402147a2fdff2a937087639ca
> # Parent  0f2bea2665040e626a7b6d039b698def3bf58dd4
> ui: implement ui.sections()
>
> An upcoming patch will introduce a configuration mechanism allowing
> configurations for different but related items (paths) to be stored
> in different config sections. There is currently no public API to
> iterate over sections or for all sections within a logical group
> of sections. So, we introduce that API.
>
> This API could arguably go on the lower-level config class. This
> should be scrutinized during review.

Doing my best impression of mpm: I think patches 1-4 should be one
series. Maybe even 1-3.