Improve markdown rendering

[bollwyvl]

Elevator Pitch

Improve the robustness of markdown label rendering.

Motivation

The current markdown rendering on labels is fairly brittle, and especially falls down when rapidly changed (i.e. wxyz.json_schema_form).

Design Ideas

Subclass everything that uses the title and/or description fields, and add/extend componentDidUpdate with the markdown method. This may well require a full theme, but that's probably okay.