Detect changes to the notes dir when Terminal Velocity is running

[seanh]

If anything in the notes dir is changed by an external tool while Terminal Velocity is running, detect the change and update the UI.

It'd be nice to be able to edit the notes dir using a file browser, external editor, etc. while Terminal Velocity is running.

Also useful if Terminal Velocity open a files in an editor asynchronously, to respond to changes made by that editor.

Also useful when running multiple instances of Terminal Velocity at once.