Added nested property editor validation support

[rasmusjp]

Hey, I've looked at the validation and I've found a way to call the validators for each property.
Mandatory and regex validation is still needed tho.

I've only tested this on a small subset of property editors but it seems to do the validation correct.

[mattbrailsford]

Interesting. Looks like a nice fix. I'm away for a week now though so will have to take a look on my return unless @leekelleher fancies testing it?

[mattbrailsford]

One initial comment just looking at it, I'd be tempted to alter the validation message that is returned to prefix like the mandatory / regex ones do, telling you which item in the nested content has the problem, this way it's easier to find the issue.