Remove unintentional minimum size of 2px for spark-splitter

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.
Revert accidental change
Merge pull request #3480 from ussuri/splitter-look-improvement

Remove unintentional minimum size of 2px for spark-splitter