add CSS example

examples/label.nim

@@ -0,0 +1,23 @@
+# nim c label.nim
+import gintro/[gtk, glib]
+import gintro/gio except Application, newApplication
+
+proc activate(app: Application) =
+  let window = newApplicationWindow(app)
+  let label = newLabel("Yellow text on green background")
+  let cssProvider = newCssProvider()
+  let data = "label {color: yellow; background: green;}"
+  #discard cssProvider.loadFromPath("doesnotexist")
+  discard cssProvider.loadFromData(data)
+  let styleContext = label.getStyleContext
+  assert styleContext != nil
+  addProvider(styleContext, cssProvider, STYLE_PROVIDER_PRIORITY_USER)
+  window.add(label)
+  showAll(window)
+
+proc main =
+  let app = newApplication("org.gtk.example")
+  connect(app, "activate", activate)
+  discard run(app)
+
+main()