D8850: Support drag and drop between shared folder view containments

[Andras Mantia]

amantia marked an inline comment as done.

REVISION DETAIL
  https://phabricator.kde.org/D8850

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Cc: broulik, plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, 
jensreuterberg, abetts, sebas, apol, mart