Enable editor when double-clicking on stamp annotation

In Firefox, double-clicking on a stamp annotation triggers text
selection (selecting the last text element in the dom before the
annotation): this triggers the logic to make annotations not interfere
with text selection, which in turns prevents the double click from
triggering the annotation editor.

This commit fixes the problem by making annotations non-selectable, so
that clicking on them does not trigger text selection. Freetext
annotations were already non-selectable, so this commit doesn't change
that. However, we need to explicitly mark text in popups as selectable.

test/integration/stamp_editor_spec.mjs

@@ -1814,4 +1814,35 @@ describe("Stamp Editor", () => {
       );
     });
   });
+
+  describe("Switch to edit mode by double clicking on an existing stamp annotation", () => {
+    const annotationSelector = getAnnotationSelector("999R");
+
+    let pages;
+
+    beforeAll(async () => {
+      pages = await loadAndWait("issue19239.pdf", annotationSelector);
+    });
+
+    afterAll(async () => {
+      await closePages(pages);
+    });
+
+    it("must switch to edit mode", async () => {
+      await Promise.all(
+        pages.map(async ([, page]) => {
+          await page.waitForSelector(annotationSelector);
+          await scrollIntoView(page, annotationSelector);
+
+          await page.click(annotationSelector, { count: 2 });
+
+          await page.waitForFunction(() =>
+            document
+              .querySelector(".annotationEditorLayer")
+              .classList.contains("stampEditing")
+          );
+        })
+      );
+    });
+  });
 });