Beginnings of the Tag/TagProvider thing: add class 'Tag' and use it throughout the library.

TagProvider will be responsile for storing tags for feeds and notify about changes made to tags (namely, renaming a tag). The library users can access only Tag, TagProvider will be hidden behind the existing interfaces.

The default behaviour didn't change: if there is no available TagProvider, the tag's label will be stored along with the collection in Akonadi.

The next step is to implement a Nepomuk-based TagProvider.
Wish me luck!