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/private/value.h

@@ -0,0 +1,43 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/gtk/private/value.h
+// Purpose:     Helper wrapper for working with GValue.
+// Author:      Vadim Zeitlin
+// Created:     2015-03-05
+// Copyright:   (c) 2015 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_GTK_PRIVATE_VALUE_H_
+#define _WX_GTK_PRIVATE_VALUE_H_
+
+// ----------------------------------------------------------------------------
+// wxGtkValue: RAII wrapper for GValue
+// ----------------------------------------------------------------------------
+
+class wxGtkValue
+{
+public:
+    // Initialize the value of the specified type.
+    explicit wxGtkValue(GType gtype)
+        : m_val(G_VALUE_INIT)
+    {
+        g_value_init(&m_val, gtype);
+    }
+
+    ~wxGtkValue()
+    {
+        g_value_unset(&m_val);
+    }
+
+    // Unsafe but convenient access to the real value for GTK+ functions.
+    operator GValue*() { return &m_val; }
+
+private:
+    GValue m_val;
+
+    // For now we just don't support copying at all for simplicity, it could be
+    // implemented later if needed.
+    wxDECLARE_NO_COPY_CLASS(wxGtkValue);
+};
+
+#endif // _WX_GTK_PRIVATE_VALUE_H_