Summary:
- fix mutual selection of elements in ax and non-ax tree (simplifies linkedNode logic as well)
- remove unneeded extraInfo attributes (focused, nonAXwithAXchild, & hasAXNode were not/no longer being used)
- use 50/50 fixed width view for trees when both are visible
Reviewed By: jknoxville
Differential Revision: D16390355
fbshipit-source-id: bbf9ea887f8f1035df8b4b0562ddcc4de291f004