mutter: sync with headerbars

- use regular raised button style for wm.

https://bugzilla.gnome.org/show_bug.cgi?id=706321
headerbar: bring inline with wm theme

- use button-like hover and active states for "wm buttons".
- lower the border-radius to match wm and top bar
- don't use a separator here for now

https://bugzilla.gnome.org/show_bug.cgi?id=706321
titlebars: lighten up

- allan suggested having icons and titles in sync,
  but making them lighter.

https://bugzilla.gnome.org/show_bug.cgi?id=706321