Fix 'connectingLine' behaviour of label

- Fix 'connectingLine' behaviour of label when label is too close to
  datapoint.
Block edit mode of label on exitState

- Block edit mode of label on exitState of 'MANY_LABEL' state.
Tweaks for IE8

- Give a space instead of blank text for IE8.

- Avoid creation of new label when mouse down on graph area for IE8.