Fixed #28 Incorrect mouse cursor shape

Fixed the correct splitter.

Explanation: 
Splitter Panel
  -- (1) Left/Top Panel 
  -- (2) h/v Splitter
  -- (3) Right/Bottom Panel```

I'm not sure how `splitter` is retrieved in `mousedown.splitter` function but it was incorrect.  

Changed to get splitter from the `Splitter Panel` i.e. second child element (2)