Improve multi-window erc-keep-place-indicator-mode

Improve multi-window erc-keep-place-indicator-mode

* lisp/erc/erc-goodies.el (erc-keep-place-indicator-follow): Describe
condition causing an indicator update.
(erc--keep-place-indicator-on-window-configuration-change,
erc--keep-place-indicator-on-window-buffer-change): Rename former to
latter, add required WINDOW parameter, and don't move indicator if
buffer appears in multiple windows.  Also, don't bother checking
whether either buffer is a mini because the manual says window change
functions don't run for minibuffer replacements.
(erc--keep-place-indicator-setup): Hook on
`window-buffer-change-functions' instead of
`window-configuration-change-hook'.
(erc-keep-place-mode, erc-keep-place-disable): Remove member from
`window-buffer-change-functions' instead of
`window-configuration-change-hook'.
(erc-keep-place): Use `visible' FRAME arg of `get-buffer-window'.
Don't twiddle `window-prev-buffers' when
`erc-keep-place-indicator-mode' is non-nil.  This feature was
originally introduced by bug#59943.
* test/lisp/erc/erc-goodies-tests.el
(erc-goodies-tests--assert-kp-indicator-on,
erc-goodies-tests--assert-kp-indicator-off): Update hook name.
* test/lisp/erc/erc-scenarios-keep-place-indicator.el: New file.
* test/lisp/erc/resources/keep-place/follow.eld: New file.