Don't draw borders wider than actual width

Rectangles passed to function xcb_poly_fill_rectangle are of type
xcb_rectangle_t and defined as:

    struct xcb_rectangle_t {
        int16_t  x;
        int16_t  y;
        uint16_t width;
        uint16_t height;
    }

The rectangles for the right and lower border had a width and height,
respectively, greater than the actual border width.

Furthermore, offset the bottom border to not overlap with the right one
and, for the top border, use r->width instead of con->rect.width as with
the other borders.
Bugfix: use the command parser to properly extract workspace names

fixes #1377