Code style overhaul

Code style overhaul
- fixed code format and a few performance / style flaws
- modernized JavaScript (always use arrow function now)
- rewrote main menu, the main menu code itself can now be restarted without having to reload the page (but other pieces of code need to be made compatible still)
- better inventory hiding / showing
- use EventTarget for some things that were previously done by callbacks
- added basic support for running as a chrome app (e.g. the quit button closing the app)
- focus more on chrome / chromium instead of firefox, fix graphical flaws caused by chromium acting different than chrome
- automatic error reporting
- gui tweaks, especially in main menu
- some terminology changes and simplification of internal structures, less polluted dragonblocks namespace
- more effective interaction with backend, do more things backend-side
- use saved DOM element references instead of IDs in some cases
- use less object orientation in cases where it does not make sense to use OOP