Simplify EventContent a bit

Main changes:
1. Base::fillJson() gets a QJsonObject& instead of QJsonObject* - c'mon,
   there's nothing inherently wrong with using an lvalue reference for a
   read-write parameter.
2. UrlWithThumbnailContent merged into UrlBasedContent. The original
   UrlBasedContent was only used to produce a single class,
   AudioContent, and even that can logically have a thumbnail even if
   the spec doesn't provision that. And there's no guarantee even for
   visual content (ImageContent, e.g.) to have thumbnail data; the
   fallback is already tested.
3. toInfoJson is converted from a template to a couple of overloads
   that supersede fillInfoJson() member functions in FileInfo/ImageInfo.
   These overloads are easier on the eye; and clang-tidy no more warns
   about ImageInfo::fillInfoJson() shadowing FileInfo::fillInfoJson().
4. Now that UrlWithThumbnail is gone, PlayableContent can directly
   derive from UrlBasedContent since both its specialisations use it.
5. Instead of FileInfo/ImageInfo, fillInfoJson() has been reinvented
   within UrlBasedContent so that, in particular, PlayableContent
   wouldn't need to extract 'info' subobject and then roll it back
   inside the content JSON object.

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