Adwaita: Move .ssd class around

Make the theme follow our documentation for the various .csd and
.ssd style classes: They all go on the window node. For now, just
add the new selector; the old one will be removed when mutter has
been updated.

https://bugzilla.gnome.org/show_bug.cgi?id=760714

gtk/theme/Adwaita/_common.scss

@@ -3557,6 +3557,7 @@ decoration {
   .popup & { box-shadow: none; }
 
   // server-side decorations as used by mutter
+  .ssd &,
   &.ssd { box-shadow: 0 0 0 1px $_wm_border; } //just doing borders, wm draws actual shadows
 
   .csd.popup & {

gtk/theme/Adwaita/gtk-contained-dark.css

@@ -4557,7 +4557,7 @@ decoration {
     border-radius: 0; }
   .popup decoration {
     box-shadow: none; }
-  decoration.ssd {
+  .ssd decoration, decoration.ssd {
     box-shadow: 0 0 0 1px rgba(28, 31, 31, 0.9); }
   .csd.popup decoration {
     border-radius: 0;

gtk/theme/Adwaita/gtk-contained.css

@@ -4698,7 +4698,7 @@ decoration {
     border-radius: 0; }
   .popup decoration {
     box-shadow: none; }
-  decoration.ssd {
+  .ssd decoration, decoration.ssd {
     box-shadow: 0 0 0 1px rgba(0, 0, 0, 0.23); }
   .csd.popup decoration {
     border-radius: 0;