Use BigRectangle to draw the border and background if there's
a border width or border radius and no border image. (Only
uniform borders are supported for now with some deviations
from the CSS model noted in the comments.)
The background color and image parameters are removed from
NbtkWidget's draw_background() method since they were not used
for NbtkButton (the only current user) and the encapsulation
break that they presented caused some minor problems.
Add a test case for borders, and also use borders to style
the buttons in the 'inline-style' test case.
