Gtk-4: Add initial suppport to gtk v4

gtk-4.0/widgets/_comboboxes.scss

@@ -0,0 +1,41 @@
+/**************
+ * ComboBoxes *
+ **************/
+combobox {
+  arrow {
+    -gtk-icon-source: -gtk-icontheme('pan-down-symbolic');
+    min-height: 16px;
+    min-width: 16px;
+  }
+
+  &.linked {
+    button:nth-child(2) {
+      &:dir(ltr) { @extend %linked:last-child; }
+      &:dir(rtl) { @extend %linked:first-child; }
+    }
+  }
+
+  &:drop(active) { // FIXME: untested
+    box-shadow: none;
+
+    button.combo { @extend %button_basic:drop(active); }
+  }
+}
+
+.linked > combobox > box > button.combo {
+  // the combo is a composite widget so the way we do button linking doesn't
+  // work, special case needed. See
+  // https://bugzilla.gnome.org/show_bug.cgi?id=733979
+
+  &:dir(ltr),
+  &:dir(rtl) { @extend %linked_middle; } // specificity bump
+}
+
+.linked:not(.vertical) > combobox:first-child > box > button.combo { @extend %linked:first-child; }
+.linked:not(.vertical) > combobox:last-child > box > button.combo { @extend %linked:last-child; }
+.linked:not(.vertical) > combobox:only-child > box > button.combo {@extend %linked:only-child; }
+
+.linked.vertical > combobox > box > button.combo { @extend %linked_vertical_middle; }
+.linked.vertical > combobox:first-child > box > button.combo { @extend %linked_vertical:first-child;}
+.linked.vertical > combobox:last-child > box > button.combo { @extend %linked_vertical:last-child; }
+.linked.vertical > combobox:only-child > box > button.combo { @extend %linked_vertical:only-child; }