Limit label position within bounds on edit

- Limit label position within bounds on edit.
Limit label within input area

- Limit label within input area.

- Update label position's score weightage.
Allow arrowhead drag

- Allow arrowhead drag to mark a different point.
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.
Fix cursor hover bug after max labels on graph

- Fix cursor hover bug after max labels on graph.
Add some checks to handle cursor

- Add some checks to handle cursor.
Handle the cut and paste event of context menu

- Handle the cut and paste event of context menu.
Add call to 'sc_super' in 'didCreateLayer'

- Add call to 'sc_super' in 'didCreateLayer'.
Add 'isHighlighted' property in label view

- Add 'isHighlighted' property in label view which is true when label
  body or arrow-head in under drag.

- Clean-up code a bit.
Update 'integration/label_view' test script

- Update 'integration/label_view' test script with arrow-head drag
  tests.
Use 'pointer-events:none' for topAnnotationsHolder

- Use 'pointer-events: none' for topAnnotationsHolder.
Fix shift of text area in iOS on keyboard slide

- Fix shift of text area in iOS on keyboard slide.
Forcefully create layer for 'editable_label'

- Create layer for 'editable_label' when there is a call to
  'didCreateLayer' without actual creation of layer.