Refactor code

- Rename function 'getActiveLabel' as 'getLabelInEditMode' in
  views/graph.js.

- Rename property 'activeLabel' as 'labelInEditMode' in
  views/label_set.js.
Fix z-order of labels on single click

- Fix z-order of labels on single click within labelset.
Fix cursor style on arrowhead drag

- Fix cursor style on arrowhead drag when it overlaps with other label.