Allow selection of language for new documents

[smathot]

When a new document is shown, there should be an option to select another language, possibly similar to the encoding panel. This should re-open the document (or rather close it and open a new one in its place) with the appropriate code-editor class and file extension. This option should be hidden as soon as the user types content, or when the document is saved to disk, i.e. it's only shown for empty, unsaved buffers.