qt: use `text/uri-list` for internal dragging instead of input items

We already handle text/uri-list, since we support dragging from external sources. However, internally, this route is not used and rather input items are considered for dragging.

This change does not affect internal intra-view move, but affects inter-view insertion.

This also makes it possible to drag media library items that are collection of media outside the application.

Based on !5899 (merged).