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/activityindicator.h

@@ -0,0 +1,60 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/activityindicator.h
+// Purpose:     wxActivityIndicator declaration.
+// Author:      Vadim Zeitlin
+// Created:     2015-03-05
+// Copyright:   (c) 2015 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_ACTIVITYINDICATOR_H_
+#define _WX_ACTIVITYINDICATOR_H_
+
+#include "wx/defs.h"
+
+#if wxUSE_ACTIVITYINDICATOR
+
+#include "wx/control.h"
+
+#define wxActivityIndicatorNameStr wxS("activityindicator")
+
+// ----------------------------------------------------------------------------
+// wxActivityIndicator: small animated indicator of some application activity.
+// ----------------------------------------------------------------------------
+
+class WXDLLIMPEXP_ADV wxActivityIndicatorBase : public wxControl
+{
+public:
+    // Start or stop the activity animation (it is stopped initially).
+    virtual void Start() = 0;
+    virtual void Stop() = 0;
+
+    // Return true if the control is currently showing activity.
+    virtual bool IsRunning() const = 0;
+
+    // Override some base class virtual methods.
+    virtual bool AcceptsFocus() const wxOVERRIDE { return false; }
+    virtual bool HasTransparentBackground() wxOVERRIDE { return true; }
+
+protected:
+    // choose the default border for this window
+    virtual wxBorder GetDefaultBorder() const wxOVERRIDE { return wxBORDER_NONE; }
+};
+
+#ifndef __WXUNIVERSAL__
+#if defined(__WXGTK220__)
+    #define wxHAS_NATIVE_ACTIVITYINDICATOR
+    #include "wx/gtk/activityindicator.h"
+#elif defined(__WXOSX_COCOA__)
+    #define wxHAS_NATIVE_ACTIVITYINDICATOR
+    #include "wx/osx/activityindicator.h"
+#endif
+#endif // !__WXUNIVERSAL__
+
+#ifndef wxHAS_NATIVE_ACTIVITYINDICATOR
+    #include "wx/generic/activityindicator.h"
+#endif
+
+#endif // wxUSE_ACTIVITYINDICATOR
+
+#endif // _WX_ACTIVITYINDICATOR_H_