Remove redundant null initializations.
Test Tags::getFavorites().
Test addMultiple() with $sync=true.
Implement Tag and TagMapper classes.

Subclassed from \OCP\AppFramework\Db\Entity and Mapper,
respectively. This will allow us to also deal with shared tags.
Add option to include tags for shared items.
In Tags::rename($from, $to), check if there already is a tag named $to.
Improve error handling.
Remove caching logic from Tags class.
Have Tags::add() return a Tag object instead of an ID (just like rename()).
Change order of hasTag() and userHasTag().
Add a public interface for the Tag class, and move [I]Tags and [I]TagMapper to \OC[P]\Tagging.