These changes add support for searching in outbox folder (fixes

These changes add support for searching in outbox folder (fixes
NB#57740).
* src/dbus_api/modest-dbus-callbacks.c:
        * (find_message_by_url): if the url prefix is merge we assume
          we're trying to retrieve a message in outbox.
        * (on_dbus_method_search): we remove the MAND: and USER: strings
          in the received folder name.
        * (add_single_folder_to_list): if folder is merge folder, and
          name is Outbox, we add it to the list of folders as
          MAND:outbox.
        * (add_single_folder_to_list): now we add MAND: prefix if the
          folder is a mandatory folder (inbox, sent, drafts, archive).
          If not, we add the USER: prefix.
* src/modest-search.c:
        * (modest_search_folder): if we search in a specific folder,
          make "outbox" folder name match any folder of type outbox.
* src/modest-tny-account-store.[ch]:
        * Added new method
(modest_tny_account_store_find_msg_in_outboxes)
          that finds a message uri (with the merge:// format) in outbox
          folders, to open it properly. This way, we can open a message
          in an outbox folder from global search emails list.

pmo-trunk-r3457