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/const_wxusedef.h

@@ -54,6 +54,7 @@ library:
 @itemdef{wxUSE_ABOUTDLG, Use wxAboutDialogInfo class.}
 @itemdef{wxUSE_ACCEL, Use wxAcceleratorTable/Entry classes and support for them in wxMenu, wxMenuBar.}
 @itemdef{wxUSE_AFM_FOR_POSTSCRIPT, In wxPostScriptDC class use AFM (adobe font metrics) file for character widths.}
+@itemdef{wxUSE_ACTIVITYINDICATOR, Use wxActivityIndicator class.}
 @itemdef{wxUSE_ANIMATIONCTRL, Use wxAnimationCtrl class.}
 @itemdef{wxUSE_ARTPROVIDER_STD, Use standard low quality icons in wxArtProvider.}
 @itemdef{wxUSE_ARTPROVIDER_TANGO, Use Tango icons in wxArtProvider.}