Declare requiredContent so other plugins can turn this off

Problem/Motivation

We are developing a CKEditor widget that does not allow the markup generated here in some of its fields. We would like this plugin's button to be turned off when inside those fields.

Proposed resolution

Let's declare requiredContent so the editor can be aware of the incompatibility and turn the button off.

      const dialogCommand = new CKEDITOR.dialogCommand('quoteDialog');
      dialogCommand.requiredContent = 'blockquote';
      editor.addCommand('listicleheading', dialogCommand);