Wrap panel process_view requests to allow multiple panels to hook without calling the view multiple times

[theojulienne]

Currently if a process_view handler in a panel calls the view function and returns something, the output is silently ignored. This means a view can be called multiple times.

This patch wraps each process_view handler up so the view is only ever called once - if any one handler returns something other than None, it is passed on to Django.

[thinred]

This is essentially true, but I have read the 'process_view' of all panels and they do not execute the view at all. Do you have a test case for that? Otherwise there is no reason to wrap the view function.

[theojulienne]

ProfilingDebugPanel uses process_view. As soon as you add that panel, views are called twice. Third party panels can also use process_view, and each one that did would call the view a third, fourth, ... time.

[mitar]

Ticket #209 seems to be duplicate of this ticket.

[jezdez]

I think this has been fixed in c9d3f34.