Added ORO_GLOBAL_SERVICE_PLUGIN macro to ServicePlugin.hpp

This PR adds `ORO_GLOBAL_SERVICE_PLUGIN` and `ORO_GLOBAL_SERVICE_NAMED_PLUGIN` macros to `ServicePlugin.hpp`. They are analogous to the existing `ORO_SERVICE_PLUGIN` and `ORO_SERVICE_NAMED_PLUGIN` macros, only that they automatically load the service in the GlobalService when the plugin is imported and refuse to load it into a TaskContext.

Note that the only other way to add a service to the GlobalService seems to be the `GlobalService::require(service_name)` method, but it seems that it is not available from scripting.