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/android/setup.h

@@ -823,6 +823,7 @@
 // Default is 1
 //
 // Recommended setting: 1
+#define wxUSE_ACTIVITYINDICATOR 1 // wxActivityIndicator
 #define wxUSE_ANIMATIONCTRL 1   // wxAnimationCtrl
 #define wxUSE_BANNERWINDOW  1   // wxBannerWindow
 #define wxUSE_BUTTON        1   // wxButton