keep copied QSObjects in memory

The old method would try to pull the object from a specific memory
address, but that address could point to garbage by then.

Also, the result from `objectWithIdentifier:` was never being returned.
Now it will be.

fixes #1716
let QSLibrarian hang onto copied QSObjects
store the identifier as the identifier instead of the string value
Before writing to the pastboard, store NSFilenamesPboardType info

The current method QS uses for writing to the pasteboard (pre 10.6) can't deal well with UTIs
Tidyups and comments on UTI/pasteboard stuff
Merge pull request #1774 from quicksilver/i1716

keep copied QSObjects in memory