GSD and headerbar transparency (#249)

* gtk3: Fix headerbar transparency on some apps

Some applications draw additional headerbar elements on top of the
main csd headerbar. This results in the headerbar appearing
non-transparent, even if transparency is enabled, as two (or more)
backgrounds with 0.97 alpha are stacked on top of each other.

Fix this by making the background transparent on every headerbar
that's descendant of another headerbar.

* gtk3: Fix titlebar separators with sidebar

This fixes separators with sidebar class on titlebars.

The issue was present at least on gnome-control-center 3.32, as
reported here: #238

common/gtk-3.0/3.18/sass/_common.scss

@@ -1091,6 +1091,11 @@ GtkComboBox {
   @extend %header_widgets;
 }
 
+.csd headerbar headerbar,
+.csd headerbar headerbar:backdrop {
+  background-color: transparent;
+}
+
 %header_separator {
   -GtkWidget-wide-separators: true;
   -GtkWidget-separator-width: 1px;

common/gtk-3.0/3.20/sass/_common.scss

@@ -1115,6 +1115,21 @@ headerbar {
   @extend %titlebar;
 }
 
+.titlebar separator.sidebar {
+  background-color: transparent;
+  border: 0 solid $header_border;
+  box-shadow: inset 0 1px lighten($header_bg, 3%);
+  margin: 0;
+
+  &:dir(ltr) { border-width: 0 1px 1px 0; }
+  &:dir(rtl) { border-width: 0 0 1px 1px; }
+}
+
+.csd headerbar headerbar,
+.csd headerbar headerbar:backdrop {
+  background-color: transparent;
+}
+
 %header_separator {
   min-width: 1px;
   min-height: 1px;