[feature][thread view] display 'non-defining' tags

[0x64746b]

I'd like to have those tags displayed with every mail that are 'non-defining' for that thread, where 'defining' tags are those that are attached to every mail in that thread. E.g.:

[inbox alot issues] sent       dtk
[inbox alot issues]             |_ pazz
[inbox alot issues] replied        |_ bruce lee
[inbox alot issues] sent                     |_ dtk

with the defining tags being put in brackets and only the other one's to be displayed

[pazz]

nice idea. You are aware that in your scenario the result of a thread search for the non-defning tag:replied would
contain the whole thread? In this sense the tag is not non-defining..

I had a prototype once that displayed a messages tags in its summary line widget and it totally cluttered the display in my opinion. But just displaying those that are not shared by all other mails might look nice.

I think one way to do this is like this:

• add a getter for shared tags in db.Thread. or changing .get_tags() to respect a boolean parameter 'shared'.
  Its a good idea to cache these results, as all msgs will poll it upon display.
• introducing a non-shared tag getter in message.Message that returns the difference of self.tags and
  self.thread.get_shared_tags()
• change widget.MessageSummaryWidget (in rebuild() I think) to display TagWidgets for self.msg.get_non_shared_tags()
  like ThreadlineWidget does for the search results