Multiple mixins are allowed of course. Allowing multiple implementations lead to too many problems due to plugin names for referring to the APIs of SimpleApiPlugins or the assets of AssetPlugins.
Hence __plugin_implementations__ has been deprecated in favor of __plugin_implementation__. The plugin subsystem will automatically copy the first implementation from __plugin_implementations__ to __plugin_implementation__ and log a deprecation warning.
Adjusted documentation accordingly. Also added docs for helpers.
