File action to open PDFs with richdocuments when files_pdfviewer is enabled

[elzody]

• Resolves: No Edit in Nextcloud Office option for PDF files #3621
• Target version: main

Summary

Since 28, when the files_pdfviewer app is enabled, there is no open to open a PDF using richdocuments. This PR fixes that, using the files API to register a file action. By default, when you click on the file, it will still use the files_pdfviewer when enabled.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Documentation (manuals or wiki) has been updated or is not required

[juliushaertl]

Small comment on the app icon, otherwise very nice and good to get in 👍

[juliushaertl]

/backport to stable29

[juliushaertl]

/backport to stable28

[juliushaertl]

@elzody Would be quite nice if we could follow up with a small cypress test for this to avoid further regressions

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