[Shotwell] Multi-threading Publish Photos

[Peter DO Smith]

I have found that when publishing photos (to Picasa in my case) that I can
no longer use the rest of Shotwell. This is a problem for me when publishing
a larger number of photos on a relatively slow broadband link. So I would
like to suggest that you multi-thread the Publish Photos function. I
appreciate that there are some consequent problems, for example you would
probably need to lock the selected records against any further change until
the publishing function has completed.
Peter