Add components missing from 1.6

Closes #239 (closed)

Also fixes #259 (closed)

Currently WIP as I can't find a manifest for the Base SDK, so there may be components I'm missing there.

This also adds a plugin for remote .ttf sources as buildstream doesn't handle just downloading and copying a file.

Edit: it turns out a plugin has just landed on buildstream master for this exact use case, however updating buildstream is blocked by migrating to CAS, so the plugin can stay, temporarily.