Make wxAnimationImpl private and get rid of wxAnimationImplType

Simplify and streamline animation classes relationship: wxAnimation is
the only public class representing an animation and it can be created by
both the native wxAnimationCtrl and wxGenericAnimationCtrl using the new
public CreateAnimation() method.

Replace wxAnimationImplType enum with more flexible type info based
check.

include/wx/gtk/animate.h

@@ -65,10 +65,8 @@ public:     // public API
 
     bool SetBackgroundColour( const wxColour &colour ) wxOVERRIDE;
 
-    static wxAnimationImpl* CreateAnimationImpl(wxAnimationImplType implType);
-
-
 protected:
+    virtual wxAnimationImpl* DoCreateAnimationImpl() const wxOVERRIDE;
 
     virtual void DisplayStaticImage() wxOVERRIDE;
     virtual wxSize DoGetBestSize() const wxOVERRIDE;