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

Subclassed from \OCP\AppFramework\Db\Entity and Mapper,
respectively. This will allow us to also deal with shared tags.
Remove two obsolete try...catch blocks.
Add option to include tags for shared items.
In Tags::rename($from, $to), check if there already is a tag named $to.
Make loading of tags from DB more explicit.
Add getTag() function for accessing of a single tag.
PHPDoc fixes as suggested by @MorrisJobke.
Merge pull request #11383 from ockham/share-tags2

Share tags