Optionally combine faces in erc-display-message

Optionally combine faces in erc-display-message

* etc/ERC-NEWS: Tell module authors that `erc-display-message' can now
combine faces.
* lisp/erc/erc-button.el (erc-button--display-error-notice-with-keys):
Ask `erc-display-message' to compose `erc-notice-face' and
`erc-error-face'.
* lisp/erc/erc-match.el (erc-hide-fools): Merge `invisible' prop
`erc-match' with existing, if present, and move body to helper for
hiding matched messages.
(erc-match--hide-message): New helper function to hide messages
regardless of match type.
* lisp/erc/erc-track.el: (erc-track-faces-priority-list): Note in doc
string that faces reserved for critical messages are always
prioritized.  Wrap :type declaration in macro helper to ensure
`erc-button' is loaded beforehand.  Otherwise calling `setopt' with
the option's default value fails.
(erc-track--attn-faces): Add new internal variable for faces that
should always appear in the mode line, at least in the default client.
(erc-track-modified-channels, erc-track-face-priority): Prepend
`erc-track--attn-faces' to `erc-track-faces-priority-list'.
* lisp/erc/erc.el (erc-send-action): Ask `erc-display-message' to
apply both `erc-input-face' and `erc-action-face' to messages.
(erc--compose-text-properties): New internal variable to act as flag
for altering behavior of `erc-put-text-property'.
(erc--merge-prop): New function copied from `erc-button-add-face' for
general internal use with any text property by all of ERC.
(erc-display-message-highlight): Set fallback face to
`erc-default-face' the symbol instead of the string.  For this to
break third-party code, callers would have to supply erroneous types
for nonexistent or undefined handlers and then explicitly check for
and depend on such misuse, which seems unlikely and therefore not
worth mentioning in etc/ERC-NEWS.
(erc-display-message): Explain how `type' param works when it's a
list.  Fix code in type-as-list branch so that it optionally combines
faces instead of clobbers them.
(erc-put-text-property): Unalias from `put-text-property', but fall
back to the latter unless caller wants to combine faces, in which case,
defer to `erc--merge-prop'.
* test/lisp/erc/erc-button-tests.el
(erc-button--display-error-notice-with-keys): Expect a combined "error
notice" face.  (Bug#64301)