[nemo-qml-plugin-email] Add isOutgoingFolder() to foldersModel

isOutgoingFolder() can be used to identify folders that makes more
sense to show some message properties instead of others(e.g show recipients
instead of sender in the sent folder).
[nemo-qml-plugin-email] Return email address for recipients with empty displayName.
Merge pull request #103 from nemomobile/outgoingFolder

Outgoing folder