Theme redux

This merge request does the following:

• Remove the gtk-application-prefer-dark-theme setting
• Add a gtk-user-theme-preference setting, which is a tristate:
  • -1/0/1 for: don't-care/light/dark
• Add a ::theme-change signal to GtkSettings that allows intercepting theme changes
• Add a default handler for that signal that respects the user theme preference (as far as we can determine dark and light themes from their name, at least)
• Add a small amount of theme information api to let apps implement their own handler:
  • gtk_theme_get_available_themes
  • gtk_theme_get_light_variant
  • gtk_theme_get_dark_variant