Integration with new Pyface Font class and Trait

[corranwebster]

At a minimum it would be nice to be able to specify fonts with Pyface Fonts - ideally we might be able to replace the Kiva fonts with Pyface fonts - this needs exploration.

[corranwebster]

Some thoughts on this goal:

• there is a fairly close mapping of the Pyface Font class to a kiva.fonttools FontQuery (there is a difference in the way that stretch is defined, and it is intrinsic because font query values are scores, but font values are size percentages)
• the Kiva Font class doesn't have the same expressiveness, so conversions from Pyface Font -> Kiva Font -> FontQuery loses information which could be used.
• we could replace Kiva Font with Pyface Font but that then means that you need Pyface, so that's a non-starter
• we could make the FontManager Pyface Font aware, but there are toolkits which don't use the FontManager (and many of them are limited in how they express font information)

So options are:

• make Kiva Font as expressive as Pyface Font (or close to) and add utilities for conversion.
• have Kiva backends accept Pyface Font objects as well and provide utilities to help interpret them.
• create an ABC or interface for font-like things and a system of adapters (care needed here, since Kiva also doesn't use Traits in its core).
• lean in to Kiva depending on Traits/Pyface (just ensure everything works with the null toolkit)