Listen for external prefs.json changes

[peterflynn]

Currently, if you edit prefs.json using an external app...

1. Brackets doesn't reflect the changes until the next restart
2. If you change any preferences within Brackets after doing the external edit, but before restarting, they will be lost

We could fix both of these by listening for prefs.json changes with a file watcher.

Implementing this involves two wrinkles:

• It means having two watch roots active at once (one for the user's project, one for the prefs storage location). The code supports this but it hasn't been heavily tested yet.
• Overlapping watch roots aren't supported currently, so to handle the case where the user's project contains the prefs folder we'd have to do extra work -- either add support for overlapping roots, or remove the prefs watchers when opening such projects and reinstate it when closing them.

So this may be MTB.

[dangoor]

Reviewed move to backlog due to the complexity that you've noted.