Extra units support for :Tag()

[p3lim]

This PR contains the following changes:

• 582d8df Minor change to avoid doing the same check twice
• 2ffcfbc 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

[haste]

I still feel that the second commit is an unnecessary complication for a very special use case.

You want to avoid having to create a frame (or custom tag function), by creating a new table on every single tag. That kinda defeats the point.

[p3lim]

I could add a check to only create the table when extra units are supplied, and change the check in the event handler to reflect that.

An alternate solution would be to allow overriding parts of the tags element (like in every other element), for example, letting the layout replace the event handler with a custom one instead of having to rewrite the entire element.

[p3lim]

Should be fine to merge, I've been using this myself for 2+ years without any issues.