Zeus rebase 02_20_13_a

This pull request contains the code for following :

- Labels created should try not to overlap other labels.

- Labels should try not to overlap axes on label creation.

- Connecting lines of labels should be long enough to show the arrow on
  label creation if possible.

- Labels should be created within graph pane and shouldn't draggable
  outside it.

- If 'enter' key is pressed when label is in editing mode, label should
  come out of the editing mode instead of adding a new line.

- Label is brought on top on single click.

- Connecting lines can be attached to labels on any of the sides
  depending on the position.