re-decoupled _ctx's stylesheet

- its mutability makes inheritance trickier
- as does Text’s unhealthy coupling to it
be more permissive w/r/t font widths

- width args can be a string-y name or (now) None
- i *believe* this is the right fix (vs putting it in the widthy()
predicate). wait and see…
refactored typography classes

- Text, Stylesheet, and StyleMixin are no longer scribbling all over
each other’s internal state
- text is now styled into an NSAttributedString at invocation-time
rather than waiting for the canvas to draw()
- text instances can be extended using the new .append() method (which
accepts all the same kwargs as the text() command)
- the `style` kwarg now just picks a named style from the stylesheet,
it no longer toggles xml-parsing (stay tuned). when combined with other
styling args, the named style is applied `first’ and the other options
are layered on top.
- the stylesheet inherited by text objects is no longer mutated as a
matter of course (so the hacky `DEFAULT` style can be ditched from the
_cascade loop)