Celiagg backend font selection is wrong on Windows

[jwiggins]

Due to Celiagg using Windows font rendering, the selection of fonts needs to work a bit differently on Windows.

With FreeType, a celiagg.Font gets passed the path to the font file being loaded (along with a face index if it's a font collection). But with Windows, the first argument of celiagg.Font is a face name that gets passed to CreateFont.

To work around this, all we need to do is use kiva.Font.findfontname() on Windows and continue using kiva.Font.findfont() on other platforms.

[jwiggins]

The foundation for this has been laid by #605, celiagg/celiagg#84 and celiagg/celiagg#85. Once a new version of celiagg has been released, some small changes to the set_font method will be needed to apply the font style.