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 2399803 by Jamie Dale in Main branch]
Made sure everything was using FText rather than FString, and also updated the property editor API to return tooltips as FText rather than FString.
[CL 2373917 by Jamie Dale in Main branch]
The display name, filter string, and tooltips for the property editor (and associated detail customizations) are now stored as FText rather than FString. This allows us to remove SLATE_TEXT_ARGUMENT from the detail customization widgets.
[CL 2372595 by Jamie Dale in Main branch]