[Libreoffice] SID_DOC_MODIFIED, booleans/strings/whatever it feels like

After much digging around, I've come to the conclusion that at some time
in the past the SID_DOC_MODIFIED property could be basically anything,
especially a string, e.g. "*" to indicate a modified doc. Then probably
there was some clean up to give it a Boolean modified/unmodified state,
with most things unified behind SvxModifyControl::RegisterControl

There was one probably unconverted remnant in basctl where a String of
"*" means modified and "" means unmodified. Which triggered a sad state
of asserts and dodgy casts which I've now fixed.

What remains is one non Boolean SID_DOC_MODIFIED in sw of

SfxStringItem( SID_DOC_MODIFIED, ' ' )

I wonder if ' ' attempts to indicate a modified document, or an
unmodified document ? I'm guessing a hold-over from a character-based
interface where ' ' overwrites a '*' and it might indicate an unmodified
document. It happens after a sw's "Save", which would fit with that.

C.