Adds basic support for writing Plugins using the lua scripting language.
Lua Plugins are registered just as native plugins are and can be enabled
or disabled via the plugin dialog.
see src/plugins/lua/README.md for further details.
Change-Id: I9f4d15e9632c46e1c6c132bcd0bbcdd70b150640
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
