OSX: Fixed bugs in presenters and services.

- SyncDeviceManagerService: Fixed bug where the discovery service would
not start.
- SyncDiscoveryService: Fixed several bugs where the discovery service
would stop scanning IP addresses.
- LibraryPreferencesPresenter: Fixed bug where the Library Service
enable/disable state would not be saved into configuration (then not
starting the service on startup)
- LoopDetailsPresenter: Fixed bug where the UpdateLoopSegmentOrder
method would block from changing segment positions.
- SegmentDetailsPresenter: Fixed bug where the markers would not be
updated before the segment details (so the Markers combo box would not
have the right value)