[Mirroring] Need static constants for layoutDirection style/property "ltr" and "rtl"

Found in 4.5.0.14124

Steps to reproduce:
1. Check LangRef for static constants for "ltr" and "rtl".

Actual Results:
Only static consts i found were TLF class:

flashx.textLayout.formats.Direction.LTR — Constant Static Property
Specifies left-to-right direction for text.

Expected Results:
We should probably have a LayoutDirection class with static enums for LTR and RTL.

Workaround (if any):
use string literals ("ltr" and "rtl") instead of consts.