Backurl "lost" when using "read" as default mode.

[toirl]

This affects the ListingFieldRenderer.

When configuring the application to open every page in readmode on default (http://ringo.readthedocs.io/en/latest/start.html?highlight=readmode#application-re)
we seem to loose the backurl functionallity.

The reason for this is

1. that the backurl is only generated if the form is not in readmode.
2. Further even if we switch into update mode and than click on a item in the Listing (with generated backurl), the backurl is lost, as the item will be again opened in readmode (because default readmode) on the context menu (top right of the item) does not render the backurl.