web: Add new widget TextDisplayLink

[mhthies]

From the discussion #89: A new widget type TextDisplayLink shall be added that behaves just like TextDisplay but at the same time, the value works as a hyperlink to another SHC WebPage or an external URL. The value should be simply contained in an <a> tag and thus be rendered as a link.

The client side implementation of the TextDisplay widget can be reused for the new widget type.

The TextDisplayLink constructor shall take an additional parameter for specifying the link destination, which can either be e full URL as a string or a WebPage object or a new WebPageRef object (as already described in #92), 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.