Backport: Using a double in plugin configuration is system locale dependent

Set up: a (service) plugin wants to use a configuration property with type Double.

When retrieving this property in the plugin using config.getDouble, where config is a ClusterConfigDecorator
implementation of IPluginConfig, the parsing of the value depends on the default system locale. So this can lead to different behaviour on different machines.

The reason for this is that Hippo's AbstractValueMap.getDouble uses Wicket's StringValue.toDouble which uses a locale from
Locale.getDefault().