We set some defaults in the FolderConfiguration struct definition. These are effectively only used for the initial "Sync" folder, because we only call SetDefaults in NewConfig and not when deserializing. The defaults for new folders are set in syncthingController.js. Defaults for new config options are never applied to existing folders.

Suggested cleanup:

• Some hack to make sure we apply defaults on unmarshalling a config from XML/JSON.
• A new REST API call .../folder-defaults that returns the default values to use to the GUI. Currently this can be just the compiled in defaults, but in the future one could imagine this being the result of compiled in defaults + user changeable ones...