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.