Version 0.4: Include style controls in GUI SVG builder.

The GUI now uses widgets to control fill colour, fill opacity, stroke
colour and stroke width. All attribute names in the code replace "-"
with "_". Commonalities in the mouse events for drawing elements are
tidied into functions.

	modified:   svgwidgets.js
	modified:   svgwidgets.py
	modified:   svgwidgets_example.ipynb