[Shared] Added ToString() overload for SettingKey

ToString()-overload is helpful for logging.
[Settings] Fixed version numbering
[Settings] Allow for batch-saving of multiple settings at once

This addresses several issues:
- Batch-saving is a little more efficient: sending a few medium-sized SOAP messages is better than sending lots (around 140 for our settings amount) messages, each including a reply from the server
- It allows for quickly determining which section has changed. This is necessary for certain clients that require this information to come in a chunk rather than independent.

Also fixed SettingsService.cs not being an ExposedCallbackService. This eventually allows clients to react on setting changes, such as the Windows UI.