added GTK4 support

examples/gtk3/builder.nim

@@ -0,0 +1,31 @@
+# https://developer.gnome.org/gtk3/stable/ch01s03.html
+# builder.nim -- application style example using builder/glade xml file for user interface
+# nim c builder.nim
+import gintro/[gtk, glib, gobject, gio]
+
+proc hello(b: Button; msg: string) =
+  echo "Hello", msg
+
+proc quitApp(b: Button; app: Application) =
+  echo "Bye"
+  quit(app)
+
+proc appActivate(app: Application) =
+  let builder = newBuilder()
+  discard builder.addFromFile("builder.ui")
+  let window = builder.getApplicationWindow("window")
+  window.setApplication(app)
+  var button = builder.getButton("button1")
+  button.connect("clicked", hello, "")
+  button = builder.getButton("button2")
+  button.connect("clicked", hello, " again...")
+  button = builder.getButton("quit")
+  button.connect("clicked", quitApp, app)
+  #showAll(window)
+
+proc main =
+  let app = newApplication("org.gtk.example")
+  connect(app, "activate", appActivate)
+  discard run(app)
+
+main()