Item14323: API change to addButton in tinymce 5.x

This is maybe a partal fix for the following JS error reported by
TinyMCE:

Failed to initialize plugin: foswiki Error: "editor.addButton has been
removed in tinymce 5x, use editor.ui.registry.addButton or
editor.ui.registry.addToggleButton or editor.ui.registry.addSplitButton
instead"

However it doesn't work.  It fails with further errors:

Failed path: (toolbarbutton) Could not find valid *strict* value for
"onAction" in { "type": "button", "title": "Typewriter text", "image":
"/pub/System/TinyMCEPlugin/plugins/foswiki/img/tt.gif" } Input object: {
"type": "button", "title": "Typewriter text", "image":
"/pub/System/TinyMCEPlugin/plugins/foswiki/img/tt.gif" }

TinyMCEPlugin/pub/System/TinyMCEPlugin/plugins/foswiki/plugin.uncompressed.js

@@ -424,7 +424,7 @@
                     this.execCommand("mceSave");
             });
 
-            ed.addButton('fwtt', {
+            ed.ui.registry.addButton('fwtt', {
                 title: 'Typewriter text',
                 onclick: function(evt) { ed.formatter.toggle('WYSIWYG_TT'); },
                 image: url + '/img/tt.gif',
@@ -440,7 +440,7 @@
                 && !foswiki.getPreference("TOPIC").match(
                         /(X{10}|AUTOINC[0-9]+)/))
             {
-                ed.addButton('fwupload', {
+                ed.ui.registry.addButton('fwupload', {
                     title: 'Upload attachment',
                     image: url + '/img/upload.gif',
                     onclick: function () {
@@ -451,15 +451,15 @@
             // else browser too old (need HTML5 FormData), or the topic is
             // AUTOINC
 
-            ed.addButton('fwinsertlink', {
+            ed.ui.registry.addButton('fwinsertlink', {
                 title: 'Insert link to attachment',
                 image: url + '/img/insertlink.gif',
                 onclick: function () {
                     getListOfAttachments(showInsertLinkDialog);
                 }
             });
 
-            ed.addButton('fwchangelisttype', {
+            ed.ui.registry.addButton('fwchangelisttype', {
                 title: 'Change bullet/number style',
                 image: url + '/img/changeliststyle.gif',
                 onClick: function() { handleChangeListType(ed); },
@@ -474,7 +474,7 @@
 
             });
 
-            ed.addButton('fwhide', {
+            ed.ui.registry.addButton('fwhide', {
                 title: 'Edit Foswiki markup',
                 image: url + '/img/hide.gif',
                 onClick: function () { handleHide(ed); }