Add spark-split-view example to top index.html + cleanup the example

Also realized that vertical directions ("up" and "down") don't work as expected. The problem must be in the way spark-split-view determins and assigns flexbox styles to the top element and distributed contents: the underlying spark-splitter works fine (see its own example). Will file a bug and address later.
Merge pull request #1085 from ussuri/activate-split-view-example

Add spark-split-view example to top index.html + cleanup the example