UETOOL-213 - Minimize Slate FString -> FText conversion (remove SLATE_TEXT_ATTRIBUTE)
This fixes any editor/engine specific code that was passing text to Slate as FString rather than FText.
[CL 2401019 by Jamie Dale in Main branch]
This change includes a large rework of the scene outliner to make it more expandable and flexible with what data it supports.
Tree items now support visitation to allow for external logic on custom columns etc without having to type check and cast.
Sorting has been overhauled to be more flexible and efficient. It now supports correct lexical comparison for numbers.
[CL 2354514 by Andrew Rodham in Main branch]
