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