That way plugins don't need to implement the SettingsPlugin mixin in order to access their data folder, which is now an injected property in OctoPrintPlugin. Plugin authors should still always use the getter since that will also make sure the folder actually does exist (lazy creation).
PluginSettings.get_plugin_data_folder has been marked as deprecated.
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.
