[KFilePlacesModel] Improve outcome of dropped places

• Use a StatJob as suggested to get the display name. This way, dropping the root of a protocol (e.g. recentlyused:/) will get a meaningful name
• Emit errorMessage to indicate to the user why a place wasn't added (failed to stat, is not a folder)
• Use KIO::iconNameForUrl which also takes into account custom folder icons

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@dfaure @ngraham

I used "Places" because that's the title of the places panel, suggestions on wording welcome.