Add erc-status-sidebar integration to erc-speedbar

Add erc-status-sidebar integration to erc-speedbar

* lisp/erc/erc-speedbar.el: Require `erc-button' atop file and don't
bother loading `dframe', which `speedbar' handles for us.
(erc-speedbar): Explain that `nickbar' is the module for this group
and library for the benefit of those who run M-x customize-group.
(erc-speedbar-nicknames-window-width): New option.
(erc-speedbar-hide-mode-topic): New option determining whether to hide
the mode and topic.
(erc-speedbar-my-nick-face): New option for determining face to use
when displaying user's current nick.
(erc-speedbar-browser): Call `erc-install-speedbar-variables'
explicitly and remove top-level `with-eval-after-load'.
(erc-speedbar-insert-target): Add parenthesized channel count after
channel name in server and channel views.
(erc-speedbar-expand-channel): Hide mode and topic depending on option
`erc-speedbar-hide-mode-topic' and pass buffer to
`erc-speedbar-insert-user'.
(erc-speedbar--nick-face-function): New internal function-valued
variable.
(erc-speedbar--highlight-self-and-ops): New function to serve as
default value for `erc-speedbar--nick-face-function'.
(erc-speedbar--on-click): Dispatch `erc-nick-popup' after trimming
status chars.
(erc-speedbar-insert-user): Revise doc string.  Call
`erc-speedbar--nick-face-function' to determine face.  Change
token for both expansion and on-click text props.  Assign
`erc-speedbar--on-click' as the mouse handler for nick items.
(erc-speedbar--buffer-options): Variable to override options locally
in speedbar buffer.
(erc-speedbar--hidden-speedbar-frame): Add variable to hold original
`speedbar-frame' before spoofing by setting to selected frame
containing window showing ERC buffer.
(erc-speedbar--emulate-sidebar-set-window-preserve-size,
erc-speedbar--status-sidebar-mode--unhook): Add function
to ensure status sidebar is showing correctly and helper to
unregister from hook on teardown.
(erc-speedbar--emulate-sidebar): Add function to control sidebar
nicknames setup.
(erc-speedbar--toggle-nicknames-sidebar): Add toggle function
for speedbar or emulated sidebar.
(erc-speedbar--ensure): Add helper function to show speedbar if it's
hidden or create one if none exists.
(erc-nickbar-mode, erc-nickbar-enable, erc-nickbar-disable):
Add new mini module.
(erc-speedbar--dframe-controlled) Add function to overwrite
`speedbar-frame-mode' as `dframe-controlled' in speedbar buffer.
(erc-speedbar-toggle-nicknames-window-lock,
erc-speedbar-close-nicknames-window): Add commands to close speedbar
window and toggle its cyclability.
(erc-speedbar--compose-nicks-face): Add helper for nicks integration.
* test/lisp/erc/erc-scenarios-status-sidebar.el
(erc-scenarios-status-sidebar--nickbar): New test.  (Bug#63595)