Turn this into a Monorepo

Did not know that it is actually a thing, but according to
https://en.wikipedia.org/wiki/Monorepo

> Google, Facebook, Microsoft, Uber, and Twitter all employ very large monorepos.

The advantages (and disadvantages) are given in the article too.

We could put most of https://github.com/AppImage into this repository to benefit from the advantages given in the Wikipedia article. It's pretty much how I like things - self-standing, all in one, no entangled dependencies between projects.

At least the __core__ AppImage components: AppImageKit, libappimage, appimaged.