Allow customizing faces for queries and non-highlighted channels

At this point, we have a possibility to customize the color of a channel (in the
tracking segment of a modeline) where somebody mentioned our name. We use the
same face as for printing our name in the message itself
(`circe-highlight-nick-face`).

This is IMHO not sufficient because new personal messages are as important as
mentions in a channel and they can be easily missed when shown in the
default (for me gray) color. I am adding a support for this.

While I am at it, I am adding a possibility to customize a color of a channel,
that doesn't mention our name but has some new activity in it.

I understand that `tracking-add-buffer` allows adding buffers to
`tracking-buffers` with face and we may utilize this feature. I believe it makes
sense for what whatever it is currently used but I would prefer to have a
possibility to apply faces when rendering (in opposite to assigning a face when
some activity happens), hence `tracking-get-face`.

By default, I am setting the `circe-tracking-channel-face` and
`circe-tracking-query-face` to `nil` and therefore they are not going
to be customized and a backward-compatibility is going to be kept for
everybody who doesn't care about this feature. Personally, I am
putting the following lines to my config.

    (setq tracking-get-face-function #'circe-tracking-get-face)
    (set-face-attribute 'circe-tracking-channel-face nil :foreground my/white)
    (set-face-attribute 'circe-tracking-query-face nil :foreground my/blue)

circe.el

@@ -110,6 +110,17 @@ See the {topic-diff} parameter to `circe-format-server-topic'."
 See `circe-fool-list'."
   :group 'circe)
 
+(defface circe-tracking-channel-face
+  nil
+  "The face used by circe-tracking to show channels with activity in modeline."
+  :group 'circe)
+
+(defface circe-tracking-query-face
+  nil
+  "The face used by circe-tracking to show query buffers with activity in
+modeline."
+  :group 'circe)
+
 ;;;;;;;;;;;;;;;;;;;
 ;;;; Variables ;;;;
 ;;;;;;;;;;;;;;;;;;;
@@ -1556,6 +1567,19 @@ PATTERNS should be the list of regular expressions."
           (throw 'return t)))
       nil)))
 
+(defun circe-tracking-get-face (buffer)
+  "Return face for a given buffer. If the buffer has already some face
+specified, use it. This covers channels where our name was mentioned. Otherwise
+decide whether the buffer is for a channel, query or else and use a face based
+on this."
+  (with-current-buffer buffer
+    (cond ((get-text-property 0 'face buffer))
+          ((eq major-mode 'circe-channel-mode)
+           'circe-tracking-channel-face)
+          ((eq major-mode 'circe-query-mode)
+           'circe-tracking-query-face)
+          (t (tracking-get-face buffer)))))
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;; Nick Highlighting ;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;

tracking.el

@@ -147,6 +147,14 @@ If set to nil, all buffers will be shown."
                  (integer :tag "Maximum"))
   :group 'tracking)
 
+(defcustom tracking-get-face-function #'tracking-get-face
+  "Function that returns face for a given buffer. It defaults to
+`tracking-get-face' which simply returns any face that is stored in
+`tracking-buffers'. Circe users may set this variable to
+`circe-tracking-get-face' instead."
+  :type 'function
+  :group 'tracking)
+
 ;;; Internal variables
 (defvar tracking-buffers nil
   "The list of currently tracked buffers.")
@@ -361,7 +369,7 @@ only return that many entries, ending with '+n'."
         (while buffer-names
           (push `(:propertize
                   ,(car shortened-names)
-                  face ,(get-text-property 0 'face (car buffer-names))
+                  face ,(funcall tracking-get-face-function (car buffer-names))
                   keymap ,(let ((map (make-sparse-keymap)))
                             (define-key map [mode-line down-mouse-1]
                               `(lambda ()
@@ -447,5 +455,10 @@ This returns STRING with the new face."
                  (propertize string 'face candidate))))
       string)))
 
+(defun tracking-get-face (buffer)
+  "Return face for a given buffer. If the buffer has already some face
+specified, use it. Otherwise, return `nil'."
+  (get-text-property 0 'face buffer))
+
 (provide 'tracking)
 ;;; tracking.el ends here