* src/engine/app/conversation-monitor/app-conversation-set.vala
(ConversationSet.add_email): Check to see if the email already exists
in the conversation, and if so merge its paths rather than just
returning.
(ConversationSet.merge_conversations_async): Simplify removing
conversations to be merged rather than manually handling it, meaning
email will be unconditionally removed rather than potentially being
conditionally removed via remove_all_emails_by_identifier.
(ConversationSet.remove_email_by_identifier): Simply update an email's
paths in the conversation if it has more than one, rather than removing
it.
* src/engine/app/conversation-monitor/app-conversation-set.vala
(ConversationSet.add_all_emails_async): Replace monitor arg with a
multiset of email paths, update call site to do the path lookup on the
caller's side instead. Add unit tests.
(ConversationSet.merge_conversations_async): Remove monitor arg with a
multiset of email paths, get paths for emails to be merged from the
conversations themselves, since they should be up to date.
(Conversation.remove_all_emails_by_identifier): Add unit tests.
* src/engine/app/app-conversation.vala (Conversation): We currently can't
easily unit test Conversation instances since the ctor requires a
ConversationMonitor argument, which is complicated. Replace the monitor
arg with a base folder (which is what it actually needs the monitor for
anyway) and remove the signals in favour of adding and modifying the
API to allow folder paths to be explicitly updated. Remove
clear_owner() method and dtor now it's useless. Update call sites. Add
unit tests to make sure the add/remove and multi-path related code
still works at least.
