Gtk-4: Add initial suppport to gtk v4

gtk-4.0/widgets/_spinner.scss

@@ -0,0 +1,27 @@
+/***********
+ * Spinner *
+ ***********/
+
+menu spinner {
+    color: $selected_bg_color;
+}
+
+/*********************
+ * Spinner Animation *
+ *********************/
+@keyframes spin {
+  to { -gtk-icon-transform: rotate(1turn); }
+}
+
+spinner {
+  background: none;
+  opacity: 0; // non spinning spinner makes no sense
+  -gtk-icon-source: -gtk-icontheme('process-working-symbolic');
+
+  &:checked {
+    opacity: 1;
+    animation: spin 1s linear infinite;
+
+    &:disabled { opacity: 0.5; }
+  }
+}