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.

include/wx/chkconf.h

@@ -433,6 +433,14 @@
 #   endif
 #endif /* !defined(wxUSE_ADDREMOVECTRL) */
 
+#ifndef wxUSE_ACTIVITYINDICATOR
+#   ifdef wxABORT_ON_CONFIG_ERROR
+#       error "wxUSE_ACTIVITYINDICATOR must be defined, please read comment near the top of this file."
+#   else
+#       define wxUSE_ACTIVITYINDICATOR 0
+#   endif
+#endif /* !defined(wxUSE_ACTIVITYINDICATOR) */
+
 #ifndef wxUSE_ANIMATIONCTRL
 #   ifdef wxABORT_ON_CONFIG_ERROR
 #       error "wxUSE_ANIMATIONCTRL must be defined, please read comment near the top of this file."