Support fsnotify to get updated on fs changes in file picker and file tree

[rcoreilly]

this appears to be the best fs event notification package for go: https://github.com/fsnotify/fsnotify -- should be pretty straightforward to plug in and get changes.

[rcoreilly]

basically working, except that it is not safe to use on mac due to significant capacity limits -- opens a file descriptor for each monitored existing file, quickly exceeding capacity: https://github.com/fsnotify/fsnotify/issues/11 there is a plan to switch to FSEvents native but not much happening at this point.

once it is working on mac too, then FileTree somehow needs to block self-generated updates to prevent duplicate updating -- basically just record path and check that.

[rcoreilly]

Still really want this -- roll up sleeves and fix fsnotify!?

[kkoreilly]

We need to resolve our plan for this at some point.