Cleaner fix for #2761 (Project tree selection incorrect after opening file
by means other than sidebar) - Clear selection in project tree if current
document not exposed in tree anywhere.

Backs out pull #2869, which made the behavior of Find in Files inconsistent
with other means of opening and didn't fix other cases of the bug.

Adds unit tests for several of these edge cases.
Merge pull request #3398 from adobe/pflynn/fix-2761

Cleaner fix for #2761 (Project tree selection incorrect after opening file by means other than sidebar)