Make draft pull-requests italic

Add new face `forge-pullreq-draft' face, which makes draft pull-requests
italic by default.

Previously all done topics were italic, but that was an unnecessary
complication.  Pending topics are already bold (and unread topics are
bold and have a box around them), so it is enough for done topics to
just not be bold.

Closes #726.

lisp/forge-topic.el

@@ -156,7 +156,7 @@ faces should not set `:weight' or `:slant'."
   :group 'forge-faces)
 
 (defface forge-topic-done
-  '((t :slant italic))
+  '((t))
   "Face used for summaries of entities with no unread or open notification.
 This face is always used together with, and takes preference over,
 a `forge-{issue,pullreq}-STATE' face and should not specify any
@@ -198,6 +198,13 @@ faces should not set `:weight' or `:slant'."
   "Face used for summaries of closed pull-requests, that weren't merged."
   :group 'forge-faces)
 
+(defface forge-pullreq-draft
+  '((t :slant italic))
+  "Face used for summaries of draft pull-requests.
+A face attribute should be used that is not already used by any
+`forge-topic-STATUS' or `forge-{issue,pullreq}-STATE' face."
+  :group 'forge-faces)
+
 ;;;; Labels
 
 (defface forge-topic-label
@@ -830,7 +837,10 @@ can be selected from the start."
      nil
      (magit--propertize-face
       title
-      `(,(pcase status
+      `(,@(and (forge-pullreq-p topic)
+               (oref topic draft-p)
+               '(forge-pullreq-draft))
+        ,(pcase status
            ('unread  'forge-topic-unread)
            ('pending 'forge-topic-pending)
            ('done    'forge-topic-done))
@@ -1291,7 +1301,9 @@ This mode itself is never used directly."
     ["" :if-non-nil forge--show-topic-legend
      (:info* (lambda () (propertize "unread"           'face 'forge-topic-unread)))
      (:info* (lambda () (propertize "pending"          'face 'forge-topic-pending)))
-     (:info* (lambda () (propertize "done"             'face 'forge-topic-done)))]))
+     (:info* (lambda () (propertize "done"             'face 'forge-topic-done)))]
+    ["" :if-non-nil forge--show-topic-legend
+     (:info* (lambda () (propertize "draft"            'face 'forge-pullreq-draft)))]))
 
 (defvar forge--show-topic-legend t)