Implement math-mode text for Windows

[freakboy3742]

matplotlib uses 2 strategies for rendering text: a simple text render, and an outline-based render that is used when the text contains math/TeX expressions.

The Windows canvas backend currently can't render those math expressions. As a workaround, toga-chart does a naive stripping of math expressions, and renders them with the simple text renderer.

The fix for this will almost certainly be on the Toga canvas side; However, the workaround in toga-chart will need to be removed to test any potential fix (and, once it is fixed, be removed permanently).