The following patch implements delayed destroy of external mime viewers. All
external mime viewers on parts whose type is a member of the list
        mm-keep-viewer-alive-types
will not be destroyed when switching to another article or to another group. 
Instead, these viewers stay open until they are explicitly closed by the user,
or until gnus is exited. Any remaining temporary files are removed when
exiting gnus.

Could someone please commit this patch, and perhaps look at a more suitable
default for mm-keep-viewer-alive-types

Thanks,
Jaap-Henk