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/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; }