Code refactoring: make the extension code ES6 compatible

[elvetemedve]

GJS has made many changes to be ES6 compatible. The extension code should follow it and use the new syntax wherever it is possible.

GJS changes are presented here: https://speakerdeck.com/ptomato/modern-javascript-in-gnome

[genodeftest]

As an example, gnome-shell 3.26.1 prints many warnings like this one:

Some code accessed the property 'STORAGE_METER' on the module 'prefs_keys'. That property was defined with 'let' or 'const' inside the module. This was previously supported, but is not correct according to the ES6 standard. Any symbols to be exported from a module must be defined with 'var'. The property access will work as previously for the time being, but please fix your code anyway.

[elvetemedve]

Yep, I know it prints a bunch of warnings, but fortunately the extension seems to be working.
I'll have more free time in November when I want to fix all of these.