html ann viewer / corpus viewer: allow to show document feature or name in the html viewer

[johann-petrak]

We currently have "Document:" at the start of the document pane. This could become Document: SOMESTRING by passing parameters to show (and thus to the corpusviewer) which would make it much easier to identify the document.

• this should be the name of a document feature or a lamda that takes the document and returns a string
• the default for this setting should be a config setting so that this will happen even without explicitly specifying the parameter