Renamed BufferPlugins -> PluginGroup
Renamed BufferPlugins -> PluginGroup

Small change in preparation for adding global plugins.

google/xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,google/xi-editor,google/xi-editor,fuchsia-mirror/third_party-xi-editor,modelorganism/xi-editor,fuchsia-mirror/third_party-xi-editor,modelorganism/xi-editor,google/xi-editor,modelorganism/xi-editor