Avoid creating objects in global scope

[adangel]

From the review https://extensions.gnome.org/review/31260:

Just noticed about new Gio.FileIcon() in convenience.js and settings in (settings.js).
Please check other files too, I may missed other ones.
You cannot create objects in global scope which is the same as init:
https://gjs.guide/extensions/review-guidelines/review-guidelines.html#only-use-init-for-initialization

We are against those because:

For example, when you fill settings which is in global scope, it won't be null after disable.
That's not good, because the object remains there without any reason.
Also the callbacks for signals can get triggered while the extension is disabled.
This is not only an error but also a security risk in some cases,
since extension is disabled on lock screen.

So objects should be created in enable() process and null out in disable() process.