* Fix#727: exposing Focus and Blur events through JSONEditor options
* Comments for onFocus and onBlur exposed events
* Fixing lint issues
* Fix for #727: Updated code for emitting onFocus and onBlur events
* Seperating FocusTarget out as a seperate class
* Fixing an issue that kept on setting the focus to the last element in the editor instead of passing it out
* Moving the add() method of FocusTracker into its constructor and renaming its remove() to destroy()
* removing a flag not needed anymore and making FocusTracker.target a required input
* updating focus tracker's focus checking condition
* Emitting onBlur callback when FocusTracker is being destroyed
* fixing lint issues
Survesh Jones