[regression] Drag and Drop broken if dragMoveEvent not implemented

It looks like a fix similar to https://codereview.qt-project.org/c/qt/qtbase/+/249086 is required for QQuickItem too.

In my application I'm not reimplementing dragMoveEvent(), yet I could receive drops from other applications with older Qt versions (I'm not sure when exactly this broke, but for sure it was working with Qt 5.6).

I've now rebuilt my application for Windows (using Qt 5.14.2 with MXE in Linux) and the drag event is shown as accepted for a fraction of a second (as soon as the drop item is entered) and then the cursor icon turns to indicate that a drop is not accepted. Implementing dragMoveEvent() and calling e->acceptProposedAction() in it restores the DnD functionality.