Switch to SlimeEngine

[timsueberkrueb]

We are supporting two different Qt/QML web engines, Oxide (for Ubuntu Touch) and QtWebEngine (for everything else). I'm not quite happy with the current implementation to handle this two different engines because it's a bit messy. So I created this new, separate library (to be reusable in other projects):
https://github.com/tim-sueberkrueb/slime-engine
It's basically just a "dirty" (or slimy :)) wrapper and it might still need some improvements/bug fixes but the core features are there.

This is also related to some Ubuntu Touch issues which will be fixed while switching to Slime Engine.