DiaRenderer: replace fill_bezier with draw_beziergon ...

With the previous API it was not possible to stroke closed Bézier curves,
leading to artifacts where lines are joined. The new draw_beziergon method
takes fill and stroke color, thus also allowing to optimize for backends
which can fill and stroke in one step (e.g. SVG, Shape, WPG?).
For symmetry the signature of draw_arc, draw_ellipse and draw_polygon
should also be adapted to take the extra fill color. The respective fill
methods are than superfluous. Also more compact files get created, usually
with a single graphic element per standard object.

As the first step the object rendering code is only adapted, but not
necessarily optimized to remove the extra "stroke call". Further commits
will optimize draw_bezier some more before going to draw_polygon