Support dragging of more than one asset at a time to add

It would be useful, especially for adding variants, to be able to drag more than one item at a time to the Resource List.

Actually I’ll tag this a bug, it should probably have supported this from the start, but a lot of functionality went into the Resource List editor so was missed. Might not support multi-select/drag of actual Resource List entries though at the moment, but that’s a slightly different feature.