Show a progressbar while creating a tarball for exporting
Submitted by Jeff F.T.
For small files there is no problem, but when you're exporting a project that has a lot of clips or heavy files (multi-hundred-megabytes for example), the UI will lock up during the tarballing operation.
This should run in the background, and we should show a progressbar (in the media library like when you import clips?)
Imported from https://bugzilla.gnome.org/show_bug.cgi?id=673730