Expose units-per-em and font metrics

[justvanrossum]

Those may be separate things:

• [x] Expose units-per-em to client and use it to compute a default line distance in the editor view
• [ ] Expose font metrics such as ascender/descender, which may vary across the designspace

In UFO, both come from font info. Variable units-per-em is not allowed anywhere that I know of.

We need to make a list of variable font info items. Blue zones are also good to have access to.

Related to #907