Tracking issue for improved FTE support

[Lord-McSweeney]

Since kjarosh's initial PRs to move most FTE properties to native data have been merged, we can start real work on FTE.

There are a couple essential things we have to implement to get FTE more-or-less fully working:

• recreateTextLine and static content (avm2: TLF improvements, implement TextBlock.recreateTextLine #24313)
• Storage of ContentElement data on ContentElementObject (avm2: Store content of ContentElement subclasses as native data #24338)
• Use FormatSpans to represent ContentElement data ( avm2: Create a FormatSpans for the text block's content element in createTextLine #24351)
• Correct breaking of lines, using lower_from_text_spans or similar
• Invalidate lines when previous lines are rebroken
• Correct rendering of TextLines using LayoutLine or similar
• Make TextLine's DisplayObject properties correct (e.g. make it a container)
• Implement hit-testing on TextLines (TextLine methods getAtomBounds, getAtomIndexAtPoint)

Additionally, we should eventually implement more of the validation checks that Flash does. These may even be necessary for later work.

• ContentElement keeps track of the TextBlock or GroupElement that it is assigned to (necessary for below changes)
• ElementFormat and FontDescription can become locked
• ContentElement invalidates its owning TextBlock's lines upon changes to elementFormat
• More?