Align TextFlowExt with future getLayoutInfo

[Jugen]

Refactored TextFlowExt to prepare for future API proposed by JavaFX PR openjdk/jfx#1596
by renaming and adding methods in TextFlowLayout to match those proposed for LayoutInfo.

When the proposed API is integrated and available in JavaFX GA the following steps can be taken:

1. Rename/refactor the TextFlowExt.textLayout() method to getLayoutInfo()
2. Then delete the TextFlowExt.getLayoutInfo() method and reference field.
3. Delete TextFlowLayout and TextFlowSpan.

Special thanks to @andy-goryachev-oracle

[andy-goryachev-oracle]

Thank you @Jugen for verifying early versions of the API! Please keep in mind that the API might also change during the review process. Hope it gets into jfx24, but no guarantees. Feel free to comment in openjdk/jfx#1596