Import Greek GSoC 2018 work

This MR is meant to be a better-than-nothing import of the Greek glyph set that has been languishing for two years now with no attention. While that work was ongoing, I loosened spacing in the base sources up a bit, making import harder. The Greek set still needs to be respaced to match.

Three points:

1. I'm skipping the existing kerning on purpose: it visibly contracts text and is therefore "fixing" spacing by applying kerning, which is a no-no. It helps that spacing needs to loosen up anyway.
2. The polytonic subset needs more attention, as it uses kerning to offset uppercase letters to the left instead of baking the needed space into the actual glyph spacing. Some lowercase letters like iotadieresis also need more spacing. A Greek speaker says that accented caps never appear next to each other typographically, so one can chose to either bake the additional space in (leading to awkward indents at the beginning of sentences) or don't and only kern against punctuation like quotes and closing parentheses. I for now opted to do nothing about it.
3. The respacing of the base sources affects Greek glyphs that reference base source glyphs, like Alpha. Their spacing needs to be updated anyway to match. I did some updating, and it looks good enough to me, but a second pass later may not hurt.

Closes #1 (closed).