Double clicking the thumbnail for a TSoftObjectPtr to a UWorld will load the level without prompting the user to save

Currently, double clicking on a thumbnail for a SoftObjectPtr to a UWorld will load that level in the level editor without checking if the current world is dirty or prompting to save. This can result in lost work. It looks like SPropertyEditorAsset::OnOpenAssetEditor needs to be updated to respect AssetTypeActions::GetValidAssetsForPreviewOrEdit, as this is where UWorld (via AssetTypeActions_World) checks the dirty state before the level is loaded.

1. Create a TSoftObjectPtr<UWorld> UProperty and expose it
2. Assign it in the editor
3. Dirty the current level, then double click the property thumbnail. Note that the level loads without prompting the user to save the current level