Use System Status Icon style class

[halfmexican]

This is a visual thing that I ran into that this PR fixes

I use a transparent panel and all panel icons use the same system-status-icon class. Other extensions override this class to provide a nice shadow to help it stand out more against lighter backgrounds. I still use the menu-button class to fix the excessive padding. I'm not sure if this should be the default, but with a solid black panel nothing looks different.

[Aryan20]

Works well. One question though, can there be a setting to enable/disable this class? Asking because some coloured icons look better without the shadow.

[halfmexican]

I think I got it working now, lemme know if anything needs to be changed

[Aryan20]

LGTM, Thanks!