added gstreamer and gboxed types support

examples/gtk3/gstBasicTutorial1.nim

@@ -0,0 +1,20 @@
+# https://gstreamer.freedesktop.org/documentation/tutorials/basic/hello-world.html?gi-language=c
+# nim c gstBasicTutorial1.nim
+import gintro/gst
+
+proc main =
+  var pipeline: gst.Element
+  var bus: gst.Bus
+  var msg: gst.Message
+  ##  Initialize GStreamer
+  gst.init()
+  ##  Build the pipeline
+  pipeline = gst.parseLaunch("playbin uri=https://www.freedesktop.org/software/gstreamer-sdk/data/media/sintel_trailer-480p.webm")
+  ##  Start playing
+  discard gst.setState(pipeline, gst.State.playing)
+  ##  Wait until error or EOS
+  bus = gst.getBus(pipeline)
+  msg = gst.timedPopFiltered(bus, gst.Clock_Time_None, {gst.MessageFlag.error, gst.MessageFlag.eos})
+  discard gst.setState(pipeline, gst.State.null) # is this necessary?
+
+main()