Support object placement

[TokisanGames]

Ability to place PackedScenes on the terrain. Probably paintable on a detail layer. Unlike #43 , these can include collision, scripts, and sub scenes, so cannot go through the particle or MMI system.

In one of my tools, I provided the feature to take a directory, and randomly place any of the scenes in that directory, eg any of these 13 tall trees, so I didn't need 13 separate detail layers.

This is so low priority it will probably never be implemented. Scatter is a much more appropriate tool and we provide a script for it to detect the terrain in /project/addons/terrain_3d/extras/project_on_terrain3d.gd