web: Add Link Button Descriptor

[mhthies]

From the discussion #89: A new ButtonDescriptor subclass named LinkButton shall be added. In contrast to the exiting button descriptor types it shall not be a WebUIConnector for dynamic functionality, but instead be rendered as a static button that works as a hyperlink to another WebPage or any external URL.

The new LinkButton shall be supported with ButtonGroup widgets, as well as ImageMap widgets and as an action button in HideRow widgets.

The LinkButton's constructor shall take a destination parameter which can either be e full URL as a string or a WebPage object or a new WebPageRef object, which contains a web page name and a reference to a section within that page, that shall be added as an anchor to the link URL.