Styles: add basics

[danirabbit]

The eventual goal here is to provide the whole stylesheet in app as Adwaita does. For now, this is only adding fallback styles, so you'll need to manually set the stylesheet to empty in Gtk.Inspector to see any difference. This is mostly a copy/paste from elementary/stylesheet but with simplifications where possible.

This doesn't cover dark styles yet, I'm planning to add that in a follow-up branch.

• Move common functions and constants from Index to _common
• Add _palette
• Add bg-color and shadow functions

Add basic styles for

• Gtk.Window
• Gtk.HeaderBar
• .osd