Add support for non-dav files

[ChristophWurst]

Called like

  const fileInfo = {
	filename: '/path/to/file.jpg',
	basename: 'file.jpg',
	mime: 'image/jpeg',
	etag: 'xyz987',
	hasPreview: true,
	fileid: 13579,
  }
  OCA.Viewer.open({
	fileinfo: fileInfo,
	list: [fileInfo],
  })

• Adjust open so it can work with a path or a given fileInfo -> fetch file info only for paths
• Get rid of url file info property and supply the non-dav URL as filename source
• Specify required file info properties
• Update readme documentation
• Adjust specific viewer extensions, if necessary
  • Render the Viewer source, if available files_pdfviewer#646
• Follow-up: get rid of unused yet required etag property

[skjnldsv]

@matiasdelellis why the laughing emoji? 😁

[skjnldsv]

/rebase

[skjnldsv]

/compile amend /