File changed detection, Issue #1939

https://github.com/processing/processing/issues/1939

This monitors the open sketch for modifications to the file that it contains and offers the user the option to reload the sketch.

There are a few issues with this implementation, due to how java.nio works.
   * The monitor detects, when files are modified, but not when they are deleted or created
   * The monitor detects any files modified in the sketch directory, not just source
   * The monitor can have a miss an update if a file was saved inside the editor and then a file was modified from outside the editor within 5 seconds (give or take)
   * There is no way for the monitor to easily detect which file was modified, so the entire sketchbook is updated