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

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