[editor] A couple of small FreeText-related fixes (PR 14976 follow-up)

- Ensure that the modified-warning won't be displayed, when navigating away from the viewer, if the user has added custom Annotations and then *removed all* of them.
 - Ensure that the *initial* editor-buttons state, i.e. the `toggled`-class, is correctly displayed in the toolbar when then viewer loads.
 - Tweak the CSS-classes for the editor-buttons, such that they use the correct focus/hover-rules (similar to the sidebar-buttons).
 - Remove a no longer accurate comment from the `BaseViewer.annotationEditorMode`-setter.
 - Address a couple of *smaller* outstanding review comments, including some re-formatting changes, from PR 14976.

src/display/annotation_storage.js

@@ -69,6 +69,10 @@ class AnnotationStorage {
    */
   removeKey(key) {
     this._storage.delete(key);
+
+    if (this._storage.size === 0) {
+      this.resetModified();
+    }
   }
 
   /**

src/shared/util.js

@@ -55,7 +55,7 @@ const AnnotationEditorPrefix = "pdfjs_internal_editor_";
 
 const AnnotationEditorType = {
   NONE: 0,
-  FREETEXT: 1,
+  FREETEXT: 3,
 };
 
 // Permission flags from Table 22, Section 7.6.3.2 of the PDF specification.