Cursor revert back to normal when dragging the splitter out of its parent window

[nchevobbe]

From https://bugzilla.mozilla.org/show_bug.cgi?id=1327984 (we should mark this bug as resolved when this issue and #84 lands in mc)

[nchevobbe]

And a quick analysis :

I think that if we set the css cursor property on the html element, it will keep the same cursor as long as the mouse button is down.
I set up a little example here : http://codepen.io/nchevobbe/pen/WRxpvO .
If you only hover the body (in red), it show you a move cursor, but if you go outside, it revert to the appropriate cursor on the element you hover.
Now, if you click on the red background body, and then, with the mouse button still pressed, move to the editor, or outside the firefox window, you still have the move cursor.
We could some thing like that by adding a class to the html element the splitter lives in (.splitter-moving , or something similar), and then change the cursor in CSS ( html.splitter-moving { cursor: resize } ) and it should work.