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.

interface/wx/activityindicator.h

@@ -0,0 +1,80 @@
+///////////////////////////////////////////////////////////////////////////////
+// Name:        wx/activityindicator.h
+// Purpose:     wxActivityIndicator documentation.
+// Author:      Vadim Zeitlin
+// Created:     2015-03-05
+// Copyright:   (c) 2015 Vadim Zeitlin <vadim@wxwidgets.org>
+// Licence:     wxWindows licence
+///////////////////////////////////////////////////////////////////////////////
+
+/**
+    @class wxActivityIndicator
+
+    Small control showing an animation indicating that the program is currently
+    busy performing some background task.
+
+    @since 3.1.0
+
+    @library{wxadv}
+    @category{ctrl}
+    @appearance{activityindicator}
+ */
+class wxActivityIndicator : public wxControl
+{
+public:
+    /**
+        Default constructor.
+
+        Use Create() to really create the control after using this constructor.
+     */
+    wxActivityIndicator();
+
+    /**
+        Constructor fully creating the control.
+
+        The arguments have the usual meanings and only @a parent is typically
+        required.
+     */
+    explicit wxActivityIndicator(wxWindow* parent,
+                                 wxWindowID winid = wxID_ANY,
+                                 const wxPoint& pos = wxDefaultPosition,
+                                 const wxSize& size = wxDefaultSize,
+                                 long style = 0,
+                                 const wxString& name = "activityindicator");
+
+    /**
+        Create the control initialized using the default constructor.
+
+        This method can be used to finish the control creation if it hadn't
+        been done already by using the non-default constructor.
+     */
+    bool Create(wxWindow* parent,
+                wxWindowID winid = wxID_ANY,
+                const wxPoint& pos = wxDefaultPosition,
+                const wxSize& size = wxDefaultSize,
+                long style = 0,
+                const wxString& name = "activityindicator");
+
+    /**
+        Starts animation of the indicator.
+
+        Does nothing if the indicator is already running.
+     */
+    void Start();
+
+    /**
+        Stops the animation of the indicator.
+
+        Notice that the animation is stopped even if Start() had been called
+        multiple times before, i.e. the calls are not cumulative.
+     */
+    void Stop();
+
+    /**
+        Returns true if the control is currently showing activity.
+
+        Returns @false initially, @true once Start() is called and @false again
+        after calling Stop().
+     */
+    bool IsRunning() const;
+};