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/generic/activityindicator.h

@@ -0,0 +1,67 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/generic/activityindicator.h
+// Purpose:     Declaration of wxActivityIndicatorGeneric.
+// Author:      Vadim Zeitlin
+// Created:     2015-03-06
+// Copyright:   (c) 2015 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_GENERIC_ACTIVITYINDICATOR_H_
+#define _WX_GENERIC_ACTIVITYINDICATOR_H_
+
+#ifndef wxHAS_NATIVE_ACTIVITYINDICATOR
+    // This is the only implementation we have, so call it accordingly.
+    #define wxActivityIndicatorGeneric wxActivityIndicator
+#endif
+
+// ----------------------------------------------------------------------------
+// wxActivityIndicatorGeneric: built-in generic implementation.
+// ----------------------------------------------------------------------------
+
+class WXDLLIMPEXP_ADV wxActivityIndicatorGeneric : public wxActivityIndicatorBase
+{
+public:
+    wxActivityIndicatorGeneric()
+    {
+        m_impl = NULL;
+    }
+
+    explicit
+    wxActivityIndicatorGeneric(wxWindow* parent,
+                               wxWindowID winid = wxID_ANY,
+                               const wxPoint& pos = wxDefaultPosition,
+                               const wxSize& size = wxDefaultSize,
+                               long style = 0,
+                               const wxString& name = wxActivityIndicatorNameStr)
+    {
+        m_impl = NULL;
+
+        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 ~wxActivityIndicatorGeneric();
+
+    virtual void Start() wxOVERRIDE;
+    virtual void Stop() wxOVERRIDE;
+    virtual bool IsRunning() const wxOVERRIDE;
+
+protected:
+    virtual wxSize DoGetBestClientSize() const wxOVERRIDE;
+
+private:
+    class wxActivityIndicatorImpl *m_impl;
+
+#ifndef wxHAS_NATIVE_ACTIVITYINDICATOR
+    wxDECLARE_DYNAMIC_CLASS(wxActivityIndicator);
+#endif
+};
+
+#endif // _WX_GENERIC_ACTIVITYINDICATOR_H_