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/doxygen/mainpages/screenshots.h

@@ -51,6 +51,7 @@ or directory, font or colour, ...) directly from the window containing them:
 These controls are considered to be less common and are defined in
 @ref page_libs_wxadv "adv" library:
 
+@appearance_brief{wxActivityIndicator,activityindicator}
 @appearance_brief{wxAnimationCtrl,animationctrl}
 @genericAppearance_brief{wxBannerWindow,bannerwindow}
 @appearance_brief{wxBitmapComboBox,bitmapcombobox}