Better integration of system fonts with TeX features

For example, consider this: `\text{Б}_{\text{Б}}`. It will be rendered ugly:

At the same time, a similar example `B_B` will look nice:

That's basically because the system fonts lack some important metrics of TeX fonts (which are currently defined by `` and `` sections of `DefaultTexFont.xml`).

We should invent a way to somehow derive these parameters for system fonts (take some gold ratio-based numbers, I don't know, make something up) and parametrize our `SystemFont` with these numbers (while keeping a sensible set of defaults). After that, we could revert stuff like #240
(who knows, maybe even declare that our system fonts `SupportsMetrics => true`?) and make the system fonts work well in this context.

We should also take a look at the modern TeX engines: how do they deal with `\text` in math context? Do they work any better?