Release 6 into master

Final release for the duration of the course DAT-255.

Make sure that before merging;
* All documentation (meetings, reviews, ~~developer documentation~~, ~~user guides~~) is in the docs folder and up to date.
* ~~The code is cleaned up, both using the formatting tool and that obsolete comments (such as fixed TODOs and uncommented code) are removed.~~
* ~~The .apk is exported again with the newest code.~~

See #47 for also merging to dev.

After merging;
* Tag the master branch as "release-6".
* (Update https://github.com/Jokab/Bullet-Inferno/releases/edit/untagged-780a415ea965723a9f5e to point to the tag)