Fix spark-splitter

@devoncarew

1. Added a direct attribute for setting the target size.
1. Added attributes for minimum and maximum target size: the initial size will be clipped to this range, if set; the splitter will not move further if one of the limits is reached while dragging.
1. Used the above to set the minimum filer tree view size so the toolbar doesn't get jumbled.
1. Propagate target size read from the preferences to spark-splitter in a Polymer-correct way.
1. Fixed: draggable region remained the same fixed 7px around the left edge of the splitter regardless of splitter size.