Add erc-track integration to erc-nicks

Add erc-track integration to erc-nicks

* lisp/erc/erc-button.el (erc-button--nick): Add `face-cache' slot.
(erc-button-add-nickname-buttons): Pass `erc-button--nick' object, if
created', as the boolean NICK-P parameter when calling
`erc-button-add-button'.  Keeping the latter function ignorant of
`erc-button--nick' is of course preferable, but some coordination is
now required to convey and use the "face cache".  We can introduce an
abstraction, like a local variable, if this becomes an issue.
(erc-button-add-button): Use `erc--merge-prop' instead of
`erc-button-add-face' to apply button faces.  Hold off on deprecating
the latter because it provides unique functionality for nesting faces.
Also, consult NICK-P if it's an `erc-button--nick' object for the
various overriding faces it knows about.
* lisp/erc/erc-nicks.el (erc-nicks-track-faces): New option.
(erc-nicks--get-face): Make generated face `:inherit' from
`erc-nicks-backing-face'.
(erc-nicks--highlight): Just return the generated face instead of
combining it with `erc-nicks-backing-face'.
(erc-nicks--highlight-button): Set the `face-cache' slot of the
`erc-button--nick' object when `track' is loaded and initialized.
(erc-nicks-mode, erc-nicks-enable, erc-nicks-disable): Add and remove
`track' integration.
(erc-nicks--reject-uninterned-faces): New function to remove faces
created by `nicks' from buttonized speakers and mentions.  Conform
to `erc-track--face-reject-function' interface.
(erc-nicks--ourps, erc-nicks--check-normals): New function and helper
for `erc-track--alt-normals-function' interface.
(erc-nicks--setup-track-integration): New function.
(erc-nicks--remember-face-for-track): New function to cache
nick faces owned by this module.
* lisp/erc/erc.el (erc--merge-prop): Add new optional parameter
`cache-fn', and when non-nil, call it, assigning the returned value to
that of the merged property.
* test/lisp/erc/erc-nicks-tests.el (erc-nicks-list-faces): Skip
the "Inherit: " button.  (Bug#67767)