Constants: Granite.STYLE_CLASS_OSD → Granite.CssClass.OSD

[danirabbit]

Todo:

• [ ] Don't use OSD style for overlaybar/tooltip. These are non-interactive widgets.
• [ ] Decide if Toast is OSD or tooltip (probably OSD. It's interactive)
• [ ] Needs demo page
  • [ ] Gtk.Button
    • [ ] Text button
    • [ ] Image button
  • [ ] Gtk.MenuButton
  • [ ] .circular button
  • [ ] Linked buttons
  • [ ] Toolbar
• [ ] Make sure we didn't break Gtk.Video controls