Add wxActivityIndicator control.

This is a simple animated control indicating some program activity.

Provide native GTK+ (for > 2.20) and OS X implementations as well as a generic
one used under MSW.

Update the sample and the documentation.

docs/changes.txt

@@ -65,6 +65,7 @@ Unix:
 All (GUI):
 
 - Allow requesting modern (3.x+) OpenGL version in wxGLCanvas (Fabio Arnold).
+- Add wxActivityIndicator.
 - Add wxWindow::FromDIP() for simpler high DPI support.
 - Allow customizing window shown by wxBusyInfo.
 - Add wxAddRemoveCtrl.