Initial RFC for Doctype Replacement Helper

[gsteel]

Q	A
BC Break	yes
RFC	yes

Description

I think the doctype helper could do with a lot of simplification and the added file DoctypeV3 is what I think the helper should look like:

• Construct expects a single string argument that identifies a doctype declaration
• There is a method that allows you to retrieve the configured doctype, or any other known doctype by its id
• Casting to a string yields the configured doctype
• There are a few other methods that can tell you if the configured or the given doctype is Html5, RDFa or or Xhtml.

Removed functionality:

• Custom DocTypes are gone (Why would you need one? and if you do, why, not just write your own helper or hard-code it in the layout?)
• The static registry thing has gone
• Run-time modification of the doctype is gone.

If something like this can be agreed in principal, I'll rename/replace the existing helper and factory to suit and write/update all the tests broken by the change.

I'll also create a pull for the 2.x series with all the deprecations.