Visual indicators when capturing onRelease #157

onRelease is a great mechanism to prevent re-rendering of the windows while resizing (this assumes you're storing the state of the windows externally).

However, because the windows are not resizing, the experience is a bit rough:

• There is no visual feedback given to the user to indicate what the potential effects of their resize operation may be
  • I'd expect a line to be drawn on top of other windows showing how the split pane is going to be placed as the cursor is moved
• Events (entering an element, hovering, etc.) still make their way to the windows underneath while resizing, causing additional side-effects that aren't desired

To mitigate these requests, it would be great if react-mosaic would do the following:

• Draw a line over the windows to represent the panes that are being resized and their final position when the operation is complete
• Prevent input events from being fired on underlying windows when the split pane is being resized