added GTK4 support

examples/gtk3/count_button.nim

@@ -0,0 +1,29 @@
+# nim c count_button.nim
+import gintro/[gtk, glib, gobject, gio]
+
+type
+  CountButton = ref object of Button
+    counter: int
+
+proc buttonClicked (button: CountButton; decrement: int) =
+  dec(button.counter, decrement)
+  button.label = "Counter: " & $button.counter 
+  echo "Counter is now: ", button.counter
+
+proc appActivate (app: Application) =
+  var button: CountButton
+  let window = newApplicationWindow(app)
+  window.title = "Count Button"
+  initButton(button, "Counting down from 100 by 5")
+  button.counter = 100
+  window.add(button)
+  button.connect("clicked",  buttonClicked, 5)
+  window.showAll
+
+proc main =
+  let app = newApplication("org.gtk.example")
+  connect(app, "activate", appActivate)
+  discard app.run
+
+main()
+