Hello. I would like to know, how to make the models (paint texture, trees, grass, …) show up in a list in an in-game editor to choose from and then be able to place and position them on the map. Most important part, how to load/add them (I probably can manage the list and placing). Maybe it would be also good to see the borders (or mesh) of the object on the cursor when picking one model so placing can be more precise. It will be for an in-game map editor. Thanks.
To load an object, if you don’t mind having them in your exe/.unity3d/etc, use Resources.Load. If you’d rather download them at runtime, use the WWW class.
To outline then, take a look there (there proabably is other ways) : http://wiki.unity3d.com/index.php/Silhouette-Outlined_Diffuse
Finally, the drag and drop from your UI, well there isn’t any magic trick but you’ll have to deal with mouse down and mouse up events.