Refactor Undo/Redo functionality

* Move major parts of undo, redo and backStore from
  www/JBrowse/View/Track/EditTrack.js to
  www/JBrowse/Store/SeqFeature/ScratchPad.ja
* Implement a Stack class to prevent overflow of
  Undo and Redo stacks

Signed-off-by: Harsh Gupta <gupta.harsh96@gmail.com>