Feature: splitters between each pair of tabs

Added splitters between each pair of design tabs so that users can drag to resize them.

### Unsolved Problem
The height of workColumns must be set to a fixed value. If it is set to 100%, it remains 0px and the workColumns won't show up. However, this disabled vertical scrolling of the page. When the screen resolution is low or when the size of browser windows is small, only part of the design tabs can be displayed. We've tried to change the type of its parent widgets to newer ones that Google suggests to use, but that only caused some unexpected behavior.

Looking forward to your suggestions. Thanks.