Convert NSString pboard types to their UTI

This fixes the query on the gGroups about the `open man page` services not working.

IMO it's not an issue with QS v1.2, but with how things have changed on OS X - Terminal advertises the service as available for `public.plain-text` whereas text entered in QS is `NSStringPboardType`.  The strings aren't equal (we were using `isEqualToString:`) hence the actions weren't showing.
There was also the added complication that the newer replacement for `NSStringPboardType` is NSPasteboardTypeString` (see `NSPasteboard.h:L292`)

Basically, what I've decided to do is to let `QSExecutor` treat both those `NS...Pboard...` types above as `public.utf8-plain-text` (`NSStringPboardType` is a 'tag' of that, according to the `System-Declared Uniform Type Identifiers` Apple doc), then do `UTTypeConformsTo` checks on those.