Better integration of system fonts with TeX features

[ForNeVeR]

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 <Parameters> and <GeneralSettings> 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?

[rstm-sf]

I correctly understood that this is a generalization of #140?

[ForNeVeR]

Yas.