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.

samples/widgets/widgets_vc8.vcproj

@@ -806,6 +806,10 @@
 			Filter="cpp;c;cc;cxx;def;odl;idl;hpj;bat;asm;asmx"
 			UniqueIdentifier="{4FC737F1-C7A5-4376-A066-2A32D752A2FF}"
 			>
+			<File
+				RelativePath=".\activityindicator.cpp"
+				>
+			</File>
 			<File
 				RelativePath=".\bmpcombobox.cpp"
 				>