[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.

web/l10n_utils.js

@@ -81,6 +81,7 @@ const DEFAULT_L10N_STRINGS = {
   printing_not_ready: "Warning: The PDF is not fully loaded for printing.",
   web_fonts_disabled:
     "Web fonts are disabled: unable to use embedded PDF fonts.",
+  freetext_default_content: "Enter some text…",
 };
 
 function getL10nFallback(key, args) {