Remove unintentional minimum size of 2px for spark-splitter

@umop

This was due to border set to be 1px on both sides irrespective to the requested splitter size. It turns out that even with `box-sizing: border-box` the 2 borders aren't merged or eliminated when the requested element's size is 1px or 0px. This commit fixes that via additional CSS rules and sets the main UI splitter size to 1px: looks a lot better.