Enlarge the surrounding of a textbox

! This example introduces the concept of the `textbox`, which is a canvas-like
structure containing the text and its surrounding path and reference point. We
demonstrate how to enlarge the surrounding box a little and use it as a
decoration of the path. ...

! The `textbox` is a specialized version of the [`box` concept
../../roadmap.html] which has been introduced into PyX in a preliminary
version. In the example, we obtain the bounding box (this is a `bbox` instance,
not to be mixed up with the `box`) and enlarge it a bit. We then draw the path
of the bounding box.

! In future releases of PyX, the boxes will have generic support for enlarging.
The syntax will then be

    tpath = tbox.enlarged(3*unit.x_pt).path()

! An alternative, geometrically different approach to enlarging the textbox path
is via an appropriate transformation, such as

    tbox = text.text(0, 0, r"Boxed text", [text.halign.boxcenter, text.valign.middle])
    tpath = tbox.path().transformed(trafo.scale(1.5, 1.5, 0, 0))