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: fsnotify/fsnotify#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.

[kkoreilly]

I disabled fsnotify on all platforms for now in #1233 since it is causing some issues; we will re-enable it once we get it all working for this issue.