Support PDF conversion triggered by collaborative tag events.

[rotdrop]

This might fix #436

[blizzz]

In workflow_script, we have similar code: https://github.com/nextcloud/workflow_script/blob/master/lib/Operation.php#L131-L143

With one difference: in this PR all nodes are being cycled through and processed, in workflow_script just the first node is being taken into account. After all, it is the same file, additional nodes might link to different representations, but it is duplicating the work.

Actually I am considering to move this logic into the framework, to some Helper or Service convenience class. This would be for 27 earliest though, as we're way beyond feature freeze for 26.

Did you decide consciously for the approach of walking through all nodes?

[Uatschitchun]

Do I get the intention of this PR correct, that adding the desired tag (maybe 'toPDF') suffice to trigger conversion?
Right now this alone doesn't suffice (just tagging an existing file without touching it).