Fix bug with unloading wxPluginLibrary objects in "wrong" order.

wxPluginLibrary objects had to be unloaded in exactly the reverse order to
which they were loaded in. This was not documented and was a serious
limitation for any realistic use of plugins anyhow, so fix it and allow
unloading them in any order now.

Instead of keeping a pointer to the last wxClassInfo not created by this
plugin, now keep a pointer to the first wxClassInfo that was created by it.
This makes the code slightly more complex but this pointer, unlike the old
one, remains valid even if another plugin was unloaded.

Closes #14261.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71571 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/dynload.h

@@ -82,8 +82,11 @@ public:
 
 private:
 
-    const wxClassInfo    *m_before; // sm_first before loading this lib
-    const wxClassInfo    *m_after;  // ..and after.
+    // These pointers may be NULL but if they are not, then m_ourLast follows
+    // m_ourFirst in the linked list, i.e. can be found by calling GetNext() a
+    // sufficient number of times.
+    const wxClassInfo    *m_ourFirst; // first class info in this plugin
+    const wxClassInfo    *m_ourLast;  // ..and the last one
 
     size_t          m_linkcount;    // Ref count of library link calls
     size_t          m_objcount;     // ..and (pluggable) object instantiations.