Introduced disabling API for the WidgetToolbarRepository class

[mlewand]

Suggested merge commit message (convention)

Feature: Introduced API to disable the WidgetToolbarRepository plugin. Closes ckeditor/ckeditor5#5964.

---

Additional information

Since this is already used in the Command class, there was an option to implement this as an interface and add a common mixin to avoid code duplication. However during an offline meeting we decided to go with easier option and just copy'n'paste it from the Command class.

[Reinmar]

I tested those changes with the below addition:

(i/5964 86ed3af M1) p@m ~/...ckeditor5/packages/ckeditor5-widget> git diff
diff --git a/src/widgetresize.js b/src/widgetresize.js
index cd1ebae..5ede5b1 100644
--- a/src/widgetresize.js
+++ b/src/widgetresize.js
@@ -80,6 +80,7 @@ export default class WidgetResize extends Plugin {
             this._activeResizer = this._getResizerByHandle( resizeHandle );

             if ( this._activeResizer ) {
+                this.editor.plugins.get( 'WidgetToolbarRepository' ).forceDisabled( 'WidgetResize' );
                 this._activeResizer.begin( resizeHandle );
             }
         } );
@@ -93,6 +94,7 @@ export default class WidgetResize extends Plugin {
         this._observer.listenTo( domDocument, 'mouseup', () => {
             if ( this._activeResizer ) {
                 this._activeResizer.commit();
+                this.editor.plugins.get( 'WidgetToolbarRepository' ).clearForceDisabled( 'WidgetResize' );

                 this._activeResizer = null;
             }

And it worked beautifully. Performance is much much better.