Add support for returning item location to wxGTK wxMimeTypesManager.

This is a squash merge of gtk_mimetype branch from
https://github.com/Hanmac/wxWidgets.git with some extra minor cleanup.

Closes https://github.com/wxWidgets/wxWidgets/pull/293

include/wx/gtk/mimetype.h

@@ -0,0 +1,32 @@
+/////////////////////////////////////////////////////////////////////////////
+// Name:        wx/gtk/mimetype.h
+// Purpose:     classes and functions to manage MIME types
+// Author:      Hans Mackowiak
+// Created:     2016-06-05
+// Copyright:   (c) 2016 Hans Mackowiak <hanmac@gmx.de>
+// Licence:     wxWindows licence
+/////////////////////////////////////////////////////////////////////////////
+
+#ifndef _WX_GTK_MIMETYPE_IMPL_H
+#define _WX_GTK_MIMETYPE_IMPL_H
+
+#include "wx/unix/mimetype.h"
+
+#if wxUSE_MIMETYPE
+
+class WXDLLIMPEXP_CORE wxGTKMimeTypesManagerImpl : public wxMimeTypesManagerImpl
+{
+protected:
+    wxString GetIconFromMimeType(const wxString& mime) wxOVERRIDE;
+};
+
+
+class WXDLLIMPEXP_CORE wxGTKMimeTypesManagerFactory : public wxMimeTypesManagerFactory
+{
+public:
+    wxMimeTypesManagerImpl *CreateMimeTypesManagerImpl() wxOVERRIDE;
+};
+
+#endif // wxUSE_MIMETYPE
+
+#endif // _WX_GTK_MIMETYPE_IMPL_H

include/wx/gtk/private/object.h

@@ -19,7 +19,7 @@ class wxGtkObject
 {
 public:
     explicit wxGtkObject(T *p) : m_ptr(p) { }
-    ~wxGtkObject() { g_object_unref(m_ptr); }
+    ~wxGtkObject() { if ( m_ptr ) g_object_unref(m_ptr); }
 
     operator T *() const { return m_ptr; }
 

include/wx/unix/mimetype.h

@@ -99,6 +99,8 @@ protected:
                        const wxArrayString& strExtensions,
                        const wxString& strDesc);
 
+    virtual wxString GetIconFromMimeType(const wxString& mime);
+
     // give it access to m_aXXX variables
     friend class WXDLLIMPEXP_FWD_BASE wxFileTypeImpl;
 };