Clean up CSS and fix issues with shadows and struts

[danirabbit]

Fixes #408

• Use a single Application.css file instead of separate application.css and panel.css (In Gtk4 we can use Granite.init to load this automatically)
• Set panel css name once instead of setting style class twice nested within itself
• Update easing func
• Remove a constants class for strings that are only used once
• Fix drop shadow not rendering when translucent styles change (for example from a solid color to a busy wallpaper)

BEFORE

AFTER

[lenemter]

This branch changed battery indicator color. Is this intentional?

[jeremypw]

Closing as linked issues appears to be fixed.

[danirabbit]

This still contains CSS fixes I'd like to get merged. I'll update the description here and address the review comment from Leo. Sorry for not updating sooner!

[danirabbit]

@lenemter fixed!