shell32: Keep at least one reference to the autocompletion object at initialization until the edit window is destroyed.

Spotted with Valgrind.

shell32: Store the autocompletion object pointer in a window property rather than in the window user data.

mshtml: Make sure previous request is stopped before starting new request in IPersistMoniker and IPersistStreamInit.

Changing the texture type has no effect at this point, and it wouldn't work
properly anyway.