Make the export-to-folder plugin less chatty.

There's no need to print when it gets activated/deacticated. None of the
other plugins do so either.

https://bugzilla.gnome.org/show_bug.cgi?id=727218
Switch slideshowshuffle to use the python3 loader.

https://bugzilla.gnome.org/show_bug.cgi?id=727222