plug-ins: add progression to PDF exporting.
PDF exporting had no progression. In particular when exporting a PDF with more a hundred pages today, it took maybe 20 secs without any progression status, which is annoying. Limitations: 1. When exporting as multi-images, we reset to 0 at each image. I am actually not sure if this is the expected behavior. 2. I am updating the progression per top layer, not doing finer grained progression steps. An extreme case would be a single top layer group with many children layers inside. Yet I wanted to avoid too much progression updates for the normal cases.
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