Mark the active Text or Image tab.

The active tab stays solid green, and the other tab is pale with green
text. A tab whose field failed validation now shows red text.

tests/js/utilities.test.js

@@ -91,6 +91,16 @@ test("showHide switches between the text and the image tabs", () => {
   assert.equal(w.showHide("other"), false);
 });
 
+test("showHide marks the tab that is showing", () => {
+  const w = publishWindow();
+  const text = w.document.getElementById("texttab");
+  const upload = w.document.getElementById("uploadtab");
+  w.showHide("upload");
+  assert.deepEqual([text.classList.contains("active"), upload.classList.contains("active")], [false, true]);
+  w.showHide("text");
+  assert.deepEqual([text.classList.contains("active"), upload.classList.contains("active")], [true, false]);
+});
+
 test("review asks for a title, then an author", () => {
   const w = publishWindow("<p>words</p>");
   assert.equal(w.review(), false);