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/gtk/activityindicator.h

@@ -0,0 +1,66 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/gtk/activityindicator.h
+// Purpose:     Declaration of wxActivityIndicator for wxGTK.
+// Author:      Vadim Zeitlin
+// Created:     2015-03-05
+// Copyright:   (c) 2015 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_GTK_ACTIVITYINDICATOR_H_
+#define _WX_GTK_ACTIVITYINDICATOR_H_
+
+// With GTK+ 3 we can always be certain that this control is available, so use
+// the normal base class. With GTK+ 2 however, we may determine during run-time
+// that we need to fall back to the generic implementation because the GTK+
+// version is earlier than 2.20, so we need to inherit from the generic class.
+#ifdef __WXGTK3__
+    #define wxActivityIndicatorGtkBase wxActivityIndicatorBase
+#else
+    #include "wx/generic/activityindicator.h"
+
+    #define wxActivityIndicatorGtkBase wxActivityIndicatorGeneric
+#endif
+
+// ----------------------------------------------------------------------------
+// wxActivityIndicator: implementation using GtkSpinner.
+// ----------------------------------------------------------------------------
+
+class WXDLLIMPEXP_ADV wxActivityIndicator : public wxActivityIndicatorGtkBase
+{
+public:
+    wxActivityIndicator()
+    {
+    }
+
+    explicit
+    wxActivityIndicator(wxWindow* parent,
+                        wxWindowID winid = wxID_ANY,
+                        const wxPoint& pos = wxDefaultPosition,
+                        const wxSize& size = wxDefaultSize,
+                        long style = 0,
+                        const wxString& name = wxActivityIndicatorNameStr)
+    {
+        Create(parent, winid, pos, size, style, name);
+    }
+
+    bool Create(wxWindow* parent,
+                wxWindowID winid = wxID_ANY,
+                const wxPoint& pos = wxDefaultPosition,
+                const wxSize& size = wxDefaultSize,
+                long style = 0,
+                const wxString& name = wxActivityIndicatorNameStr);
+
+    virtual void Start() wxOVERRIDE;
+    virtual void Stop() wxOVERRIDE;
+    virtual bool IsRunning() const wxOVERRIDE;
+
+protected:
+    virtual wxSize DoGetBestClientSize() const wxOVERRIDE;
+
+private:
+    wxDECLARE_DYNAMIC_CLASS(wxActivityIndicator);
+    wxDECLARE_NO_COPY_CLASS(wxActivityIndicator);
+};
+
+#endif // _WX_GTK_ACTIVITYINDICATOR_H_