Ensure the slideshow is stopped on plugin detach

* src/g4wihelp.c (DllMain): Stop slideshow on detach.
* src/inst-final.nsi (SEC_final): Do not stop slideshow.

If the g4wihelp is detached but the timer / windowmessages are
not stopped our timer would trigger an access violation.
This fixes crashes during installation if the g4wihelp is detached
before the slide_stop was called from SEC_final.