[edition] Add a FreeText editor (#14970)

- add a basic UI to edit some text in a pdf;
- an editor can be moved, suppressed, cut, copied, pasted, selected;
- add an undo/redo manager.

test/test.js

@@ -950,6 +950,7 @@ async function startBrowser(browserName, startUrl = "") {
       // Avoid popup when saving is done
       "browser.download.always_ask_before_handling_new_types": true,
       "browser.download.panel.shown": true,
+      "browser.download.alwaysOpenPanel": false,
       // Save file in output
       "browser.download.folderList": 2,
       "browser.download.dir": tempDir,