Feature: Add ability to collapse splitter views.

[Kilowhisky]

Add ability to collapse splitter views on command via a property.

There is an incoming PR with this feature completed.

Here's how it should work.

User controls visibility of splitter-views via a property like isVisible on the splitter view.
When a view is hidden the other view grows to fill the space.
When the hidden view is re-shown it should assume its old width.
When hidden a css class is applied to the container with either trail-hidden or lead-hidden.