added threading examples

examples/gtk3/thread2.nim

@@ -0,0 +1,38 @@
+# nim c --threads:on --gc:arc -r thread2.nim
+
+import gintro/[gtk, glib, gobject, gio]
+from  os import sleep
+
+var workThread: system.Thread[void]
+var button: Button
+
+proc idleFunc(i: int): bool =
+  button.label = $i
+  return SOURCE_REMOVE
+
+proc workProc =
+  var countdown {.global.} = 25
+  while countdown > 0:
+    sleep(1000)
+    dec(countdown)
+    idleAdd(idleFunc, countdown)
+
+proc buttonClicked(button: Button) =
+  button.label = utf8Strreverse(button.label, -1)
+
+proc activate(app: Application) =
+  let window = newApplicationWindow(app)
+  window.title = "Countdown"
+  window.defaultSize = (250, 50)
+  button = newButton("Click Me")
+  window.add(button)
+  button.connect("clicked",  buttonClicked)
+  window.showAll
+  createThread(workThread, workProc)
+
+proc main =
+  let app = newApplication("org.gtk.example")
+  connect(app, "activate", activate)
+  discard app.run
+
+main()