Clean up validation/defaults handling

[InfusOnWoW]

We have in WeakAuras a per regionType table of "defaults", which we really use for two purposes.
a) Actually as defaults for new regions
b) As a type check for validation

This has lead to problems multiple times, so we should fix that by:
a) Either having a more explicit struture, e.g.

local defaults = {
  icon = {
    type = "bool",
    default = true
  },
  [....]
  subRegions = {
     type = "subRegion", -- ??? Special logic for handling subRegions in validation?
     default = {
       [1] = "subbackground",
       [2] = "subforeground"
    }
  }
}

b) Or two separate structures

In the same are there's trap. We do validation before we do modernization. The reason for that it was that way forever. I think that's done to make Modernize safer, since it can work on validated data. But this order has also lead to problems multiple times. So we should do imho two things:

a) Move validation to be after Modernization
b) Wrap Modernize in a pcall so that if it fails for an aura, we can print a error and continue.