Translations: incomplete messages in pot file

[Mike Massonnet]

Hi,

The translation of the following messages doesn't appear entirely in the pot file:
- postler/postler-composer.vala +189
- postler/postler-compoter.vala +319

The code snippet is the following for the first message:
                 Gtk.MessageType.ERROR, Gtk.ButtonsType.NONE,
                _("Folders for sent messages and outbox can't be created. "
                + "You need to fetch mail at least once.\n"
                + "If you see this error after fetching, right-click "
                + "the folder for sent messages, \"Use as...\" and "
                + "select \"Sent\". Do the same for the outbox."));

and the pot file only grabs the first string, not the concatenated ones:
 673 #: ../postler/postler-composer.vala:189
 674 msgid "Folders for sent messages and outbox can't be created. "
 675 msgstr ""

So either use several times _() + _() or put everything in one line I guess.

Thank you,
Mike