Allow reordering lists by drag and drop

[CelticMinstrel]

Terrain sets are always listed in the order they were defined. This can get confusing if you define a lot of terrain sets. Although it's technically possible to reorder them by duplicating a set and then deleting the original, that's prone to error.

It would be nice if you could simply drag and drop to reorder them.

[bjorn]

There already was #1603 open about reordering in general, and buttons for reordering are implemented in #4388 (pending review). But indeed reordering by drag-n-drop would be nice. This is already supported for layers, tiles and animation frames, which could be looked as an example.

[CelticMinstrel]

Whoops, looks like I didn't search well enough (or forgot to search) before opening this issue! But that said, I do think drag-and-drop would be nicer than buttons, and this could in theory also be applied to the layers list and possibly other lists.

[bjorn]

and this could in theory also be applied to the layers list

So, all the lists I mentioned already support reordering by drag-n-drop, including layers list.

It's not supported in list properties though, and it may be a little more difficult to add it there now due to the widget-based approach. But it would still be nice to have it there too.