Fixed a RoundedLine Shape bug.

Also organized the RoundedLine generation code better, fixed some
comments, made the ShapeType selection text always say "Shape Type"
(rather than displaying the chosen ShapeType, which is not necessary
because the associated icon is displayed), attempted to create a
HistoryItem when the shape type is switched (which didn't work properly
with undo/redo, so I commented it out for now), and added a separator to
the MagicWandTool toolbar.