Constructor for a custom button.
- editor (HtmlEditor)
- Reference to the HtmlEditor.
- name (String)
- Name of the button.
- imageSrc (String)
- Url of the button image
- toolTip (String)
- Tooltip for the button.
- onClick (String)
- Client-side method to invoke on click.
- externalMethod (Boolean)
- True if onclick method is external to the editor (i.e. on the page).