Qt: dragging large collection of media often fails

Dragging should not start until it what to drag is determined (through MIME data) for external drags, and dragging should either not start until what to drag (resolving input items) is determined or should start but wait until drop (somehow within drag item, not sure if possible) until the input items are resolved for internal drags.

Since we need to care about external drags, and that we can't know beforehand if the drag is going to be internal or external, the timer in drag item that forces drag start should not consider the time until the data is resolved (so the MIME data is set) in the worst case. And it should not consider the time until the input items are resolved if the drag operation is unable to wait for completion of resolving input items on drop.

Origin of bug: !3908 (diffs, comment 451979)