Provide a way for MouseArea to grab mouse from parent Flickable in QML

When UI content doesn't fit fully on the view it's often put inside Flickable, which provides a way to scroll the content. For implementing the scrolling behavior, Flickable grabs mouse move events from the content items. This creates problem for implementing dragging use cases inside the content items, use cases like drag&drop item arranging or copy&paste text selection handles. MouseArea's drag.target goes around this by subsequently grabbing the mouse from Flickable when the drag starts. Unfortunately if the drag use case is too complex for drag.target approach there is no way to implement mouse grabbing from QML. QML applications can go around this by disabling Flickables they use by setting Flickable::interactive property to false during dragging, but this is not a solution for QML-based component libraries that don't have access to or knowledge of all Flickables in the parent chain, and even if they did such strong coupling would not be desirable.