Don't use auto-release pool in string-related methods

[kvark]

We have set_label methods, which use nsstring_from_str, which uses auto-release pool. This is inviting for leaks in user code. There is no reason why set_label should use any auto-release pools, technically.

Proposal - split nsstring_as_str into separate methods for creation of an NSString and borrowing it as str. This way, the calling code is responsible for freeing it when needed. don't autorelease NSString. Instead, return an object that the caller can free explicitly by dropping. The caller is our own method in this case, so we can ensure the lifetime is correct.