Fix frequent project tree flickering & scroll pos loss: effectively revert
to master's behavior where tree is only refreshed manually by user (it's
still incrementally updated due to changes within Brackets, but updating to
pick up external changes - which is always a full, flickery refresh - never
occurs automatically).

We'll reintroduce auto-tree-refresh once we can bring down the false-
positive rate for fs change events (once we have real watchers instead of
just a window-focus listener).