added gstreamer and gboxed types support

examples/gtk3/sourceview_test.nim

@@ -0,0 +1,16 @@
+import gintro/[gtk, glib, gobject, gio, gtksource]
+
+proc appActivate (app: Application) =
+  let window = newApplicationWindow(app)
+  window.title = "GTKSourceView"
+  window.defaultSize = (250, 350)
+  let view = newView()
+  window.add(view)
+  window.showAll
+
+proc main =
+  let app = newApplication("org.gtk.example")
+  connect(app, "activate", appActivate)
+  discard app.run
+
+main()