display per message subjects in thread widget

Many senders to not quite grasp the concept of a mail thread, and this
"hijack" a thread by replying and changing the subject. In addition,
there may be good reasons for replies with changed subject, such as an
adjustment of status or the typical patch series threads with individual
topics.

Currently, all this information is hidden in collapsed view but may be
valuable for expanding the right message. Therefore, provide individual
subjects for the messages in thread view.

alot/db/message.py

@@ -68,6 +68,7 @@ def __init__(self, dbman, msg, thread=None):
             self._from = '"{}" <{}>'.format(acc.realname, str(acc.address))
         else:
             self._from = '"Unknown" <>'
+        self._subject = decode_header(msg.header('Subject'))
 
     def __str__(self):
         """prettyprint the message"""
@@ -132,6 +133,9 @@ def get_message_parts(self):
             if not msg.is_multipart():
                 yield msg
 
+    def get_subject(self):
+        return self._subject
+
     def get_tags(self):
         """returns tags attached to this message as list of strings"""
         return sorted(self._tags)

alot/widgets/thread.py

@@ -78,9 +78,12 @@ def _render_wrap(size, focus=False):
     def __str__(self):
         author, address = self.message.get_author()
         date = self.message.get_datestring()
+        subject = self.message.get_subject()
         rep = author if author != '' else address
         if date is not None:
             rep += " (%s)" % date
+        if subject:
+            rep += ": %s" % subject
         return rep
 
     def selectable(self):