UX: friendly numbers in settings: Zoom factor (percent better than decimal factor)

[pano9000]

As reported in #806

Interesting thing is: In the global Menu, we already use percentage and a nice UI to controll that → why not reuse in the main settings as well?

would ideally require refactoring out the "zoom-container" from global_menu.ts into its own "component" and then we can reuse that whereever needed.

(btw: I just wanted to test the new "sub-issue" feature of Github :-))