Merge branch 'chromium-refactor', closes #1798.

This implements some longstanding work of getting the Chromium
derivation more modular. Unfortunately, I didn't manage to decrease the
compile time, which was one of the primary goal for doing the refactor.
A main reason this didn't work out well was the fact that most bundled
libraries are so heavily patched that it's not possible within a limited
time frame to decouple it from the main derivation.

However, it should now be easier to build other derivations that build
upon Chromium, like libcef. Also, it finally adds support for the
non-free PepperAPI Flash and PDF plugins and support for fetching the
corresponding versions through the updater.