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/private/animate.h

@@ -0,0 +1,44 @@
+/////////////////////////////////////////////////////////////////////////////
+// Name:        wx/private/animate.h
+// Purpose:     wxAnimationImpl declaration
+// Author:      Robin Dunn, Vadim Zeitlin
+// Created:     2020-04-06
+// Copyright:   (c) 2020 wxWidgets development team
+// Licence:     wxWindows licence
+/////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_PRIVATE_ANIMATEH__
+#define _WX_PRIVATE_ANIMATEH__
+
+// ----------------------------------------------------------------------------
+// wxAnimationImpl
+// ----------------------------------------------------------------------------
+
+class WXDLLIMPEXP_CORE wxAnimationImpl : public wxRefCounter
+{
+public:
+    wxAnimationImpl() {}
+    virtual ~wxAnimationImpl() {}
+
+    virtual bool IsOk() const = 0;
+    virtual bool IsCompatibleWith(wxClassInfo* ci) const = 0;
+
+    // can be -1
+    virtual int GetDelay(unsigned int frame) const = 0;
+
+    virtual unsigned int GetFrameCount() const = 0;
+    virtual wxImage GetFrame(unsigned int frame) const = 0;
+    virtual wxSize GetSize() const = 0;
+
+    virtual bool LoadFile(const wxString& name,
+                          wxAnimationType type = wxANIMATION_TYPE_ANY) = 0;
+    virtual bool Load(wxInputStream& stream,
+                      wxAnimationType type = wxANIMATION_TYPE_ANY) = 0;
+
+    // This function creates the default implementation for this platform:
+    // currently it's wxAnimationGTKImpl under wxGTK and wxAnimationGenericImpl
+    // under all the other platforms.
+    static wxAnimationImpl *CreateDefault();
+};
+
+#endif // _WX_PRIVATE_ANIMATEH__