qml: move drag item to a separate window

this eliminates inherently racy grabToImage mechanism

alternative to !6189 (closed), one major benefit over MR 6189, is that this keeps visual implementation in QML and provides better possibility of what can be achieved visually in drag item. This has easier implementation from our side but the underlying implementation it uses could be more flaky in edge cases, In contrast, 6189 is much more resilient but limited.

review request @chub