FIX Don't escape values on TreeDropdownField readonly views

They typically output TreeTitle() which is assumed to be HTML.
Revert "FIX: Strict error warnings on DataExtension"
This reverts commit 1960df8bc30ecb0467a33275810fd121a0125733.

Revert "FIX: validate doesn't take var by reference"
This reverts commit 866bb0713b9918e0450a516da9b7662c3287e24f.

@ajshort has changed the method signatures in 1f6f7f08. While it wasn't explicitly noted in the commit message, I think its a good change - objects like a FieldList are always passed by reference in PHP, no need to declare that behaviour.