Improved rendering of text

Text rendering (Window.Text) has been greatly
improved. This should make life easier while
waiting for fully Latex assisted formatting
(feature to be added in the next future). This is
what we can do now:
- subscripts, superscripts and newlines can be
inserted using the latex syntax (text_{sub},
text^{sup} or a^1, a_1 and the character \n);
- text is treated as all the others ordinary
paths: you can fill it, you can stroke its
contours, etc.
- text can be placed very effectively by
specifying a point relative to the text an the
destination of this point (where it should go).
For example,

w.Text["a^2", (10, 10), .From[(1, 1)]]

draws the string a^2 so that the upper-right
corner of its bounding box is over the point
(10, 10). Then, to center the text, just use:
.From[(0.5,0.5)]

We are now quite close to the first beta release
of Box: we just need to add better facilities to
trace arcs, proper clipping and interface to latex
for inserting formulas.