bug#63595: 30.0.50; ERC 5.6: Add buffer-list and nick-list modules

[J.P.]

This may come as a shock, but problems exist in the `bufbar' changes
recently introduced (by me) to erc-status-sidebar.el.

The first involves the new boolean option `erc-status-sidebar-singular'.
No matter what its doc string says, the purpose of this option remains
difficult to describe and its behavior hard to predict. This makes it
only really practical as an internal flag, which is what it should have
been from the get-go and how it's been repurposed in the attached patch.
Initially, this option was intended to alter the behavior of the
minor-mode toggle `erc-bufbar-mode' so that when the option is nil,
toggling the mode in a frame where the sidebar is hidden would summon
it, regardless of the value of the mode's variable. This kind of extreme
DWIM'ness may suit normal, standalone minor modes, but users likely
expect mode toggles for global ERC modules to work more or less
traditionally.

A somewhat related problem stems from my altering the original behavior
of existing status-sidebar commands, like `erc-status-sidebar-open', to
accommodate and promote `erc-bufbar-mode' as a unified entry point. In
retrospect, that probably threatens to anger the rare person who's
become accustomed to those commands over the past couple years and who
may not be interested in the new module at all (for whatever reason).
I've attempted to rectify this in the attached patch so that the newer
behavior only takes effect when `erc-bufbar-mode' is active. This means
users of the module must now issue an M-x erc-status-sidebar-open RET to
see the sidebar in (additional) frames that don't yet have one, but
non-module users will still only ever encounter a single sidebar
instance in an Emacs session.

The last issue involves the sidebar intermittently clobbering the
window's scroll position. I've tried to eradicate this while also
improving the overall responsiveness by having it refresh when users
select another window. Basically, updates were initially tethered to
mode-line changes, which often left users with the wrong buffer
highlighted whenever updates were few and far between. Although this is
now much improved, the change likely adds some overhead, so I've
included an escape hatch while we investigate further.

0001-5.6-Simplify-multi-frame-behavior-of-erc-bufbar-mode.patch
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