Change default icon size to 20 so they aren't defacto invisible

[ahydronous]

Some users their icons are "invisible" because the default size is set to 0. I chose size 20 because it makes most icons approximately the same size as the built-in top bar icons.

See: https://github.com/ubuntu/gnome-shell-extension-appindicator/issues/382

[DanHolli]

size of zero is supposed to be an auto size as it functions in Ubuntu, it may be better to submit an issue and figure why yours isn't auto sizing