Remove SvgPath in favor of a Knockout binding handler.

Also refactor CSS to eliminate all duplicate imports.  Shared styles are in shared.css.  Individual widget CSS files, e.g. BaseLayerPicker.css, no longer import anything.  Users are expected to import "widgets.css" to get everything in the common case, or import all of the individual stylesheets, including shared, for all widgets that they use, in the uncommon case.

This CSS organization is what other widget libraries use, specifically Dojo.