added GTK4 support

examples/gtk3/app0.nim

@@ -0,0 +1,16 @@
+# app0.nim -- minimal application style example
+# nim c app0.nim
+import gintro/[gtk, glib, gobject, gio]
+
+proc appActivate(app: Application) =
+  let window = newApplicationWindow(app)
+  window.title = "GTK3 & Nim"
+  window.defaultSize = (200, 200)
+  showAll(window)
+
+proc main =
+  let app = newApplication("org.gtk.example")
+  connect(app, "activate", appActivate)
+  discard run(app)
+
+main()