Having the plugin commands on the first level of --help proved to be
a bad idea since it basically kills every chance of lazy loading the (expensive)
plugin context. Using a sub command for anything plugin related allows us
to only fire up the plugin context if a plugin command is expected, saving us
some precious seconds of operation in all other cases.
For conformity reasons the dev sub commands were now restructured
similarly.
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.
