Reduce ref/deref operations by introducing swap() functions.

[TsudaKageyu]

Removed redundant ref/deref operations which make the memory management complicated from ByteVector, String, List and Map by introducing swap() functions.
Now, each class has only a pair of ref/deref except for ByteVector. ByteVector is a little more complicated due to the nested RefCounters should be fixed in another patch.

[lalinsky]

Did you measure the results in some way? Is there a benefit in doing this? I'd really like to focus more on tagging files and less on building a string/container library. Sorry, you have been spending a lot of time working on this part of TagLib and I feel it's getting a little too complicated, to justify being scraped and replaced by boost, if we need to continue extending it.