Add the ability to allocate extra per-dialog-instance private data

Add the ability to allocate extra per-dialog-instance private data
in the portable dialog interface. This has allowed me to remove
`ssd->savedsession' in config.c, which was (I believe) the only
out-of-place piece of per-instance data in the dialog template
stuff. Now we should actually be able to run more than one config
box in the same process at the same time (for platforms that'll find
that useful).

[originally from svn r2925]