Common: add diff, union and intersection functions for (hash)tables
ASSTags: finish deduplication, rename to cleanTags to reflect expanded scope; introduce ASSTagList which provides a number of tools to work with tag lists; add equal() method to tags that allows to compare the relevant properties of tag objects; add get() method to ASSClipVect as it is required by the equal method; ASS.instanceOf(): return first classname when called without class parameter