added foreground colored stroke/pen support

[sp1ritCS]

When the alpha of a stroke is -INFINITY, it'll render the stroke with the foreground color from the GtkStyleContext.

The black (0.0, 0.0, 0.0, 1.0) default pen color has been replaced with a new button that is able to select that particular "color". If a user has modified his toolbar colors, he'll have to drop/change that color manually.