Extra units support for :Tag()

This commit contains the following changes:

* 582d8dfe153e4cf8d4195cf05a2a55bef3f71454 Minor change to avoid doing the same check twice

* 2ffcfbc12e516ede3b2b7d5ba5b7860cfc5ffaef This commit allows the layout to specify additional units which the event handler can match against. This is to make sure you could get an update for a tag on one unit for a different unit. As an example; I have my pet's health on my player frame, as I don't see a point of having a whole frame for it. With this I can tag like this: `frame:Tag(fs, '[pethp][othertag]', 'pet')` instead of forcing `UNIT_HEALTH_FREQUENT` or other events to be "unitless" with [oUF.Tags.SharedEvents](https://github.com/p3lim-wow/oUF/blob/1.6.6/elements/tags.lua#L697)