Extend the code generator to handle GstMiniObject and implement Gst.MiniObject

Extend the code generator to handle GstMiniObject and implement Gst.MiniObject

Also fix indention everywhere and change code to use Gst.MiniObject.GetObject()
instead of GLib.Opaque.GetOpaque().

It's currently not possible to implement or use GInterfaces on mini objects
but apart from that this should be a great improvement, especially new
mini object classes can be defined in C# now.