TEMP Fixing vertical spark-split-view - DO NOT MERGE

This is a half-finished attempt to fix #1086. 

This fixes the vertical split-view in the example (widgets/examples/spark_split_view), but semi-breaks the horizontal one (it assumes the minimal possible height and 100% width, unlike before) and most importantly it breaks split-view in Spark (file tree panel disappears). Investigate further.

@sunglim It would be great if you could take a look and see if you can complete this fix (or come up with a different one). It has a lot to do with CSS, though, so no pressure :)