ConnectionSettingsDiscovery should return list of in/out settings

Creates a new data class: DiscoveryResults that holds a list of incoming
and outgoing DiscoveryServerSettings. This commit also creates a wrapper
for the currently used ProvidersXmlDiscovery to convert the new
DiscoveryResults object into ConnectionSettings.

In the future, this process will have a less-lazy algorithm that will be
smart enough to handle more than one incoming/outgoing server but this
commit assumes there are still one of each (by assertion).

Use enum to determine discovery target

Sort SrvServiceDiscovery results using pickMailService