Restore helper_alternate_class() function

This is a temporary measure to improve backwards compatibility of 1.3,
the function will be removed again in the next release.

Fixes #17522 (see also issue #17437)
New DEPRECATED error type

Make use of E_USER_DEPRECATED instead of relying on WARNING for
deprecated functionality. Allows for differentiated error messages and
handling.

Fixes #17837
Add DEPRECATED warning to helper_alternate_class()
Delay printing of deprecated warnings to page bottom

This avoids display errors when a deprecated warning is triggered from
within an HTML tag, as was the case when using helper_alternate_class().
Add link to PHP constants for unhandled error types
Reinstate helper_alternate_class() function

Pull request #234