This solves a wrapping issue where STextBlock would incorrectly consume new-line characters when wrapping, as well as allowing it to use custom wrapping behavior (eg, CamelCase wrapping for asset names in the content browser).
This change also implements FString -> FText passthrough for SLATE_TEXT_ATTRIBUTE (it was previously performing FText -> FString passthrough, however this didn't fit well with the fact that FText has a more efficient method of comparing whether a bound text attribute has changed (see FTextSnapshot), so can quickly work out that the FTextLayout doesn't need updating).
This change adds FTextBlockLayout to handle the text layout cache for text block types (STextBlock and SRichTextBlock) and serves a dual purpose of moving the duplicated caching logic into a single place, as well as hiding it from the widget (so allowing the cache to mutate, even when called from immutable widget functions).
This change also increases the unification between the STextBlock and SRichTextBlock construction parameters, by allowing STextBlock to specify a Justification, Margin, and LineHeightPercentage, and allowing SRichTextBlock to specify its HighlightText as an attribute.
ReviewedBy Nick.Atamas, Justin.Sargent
[CL 2280351 by Jamie Dale in Main branch]
