Basic ideas is:

• Support drop-box gizmo-group that is visible while dragging.
• With that, show a bounding box and a grid while dragging.
• The object always snaps with its bottom, axis-aligned bounding box edge (Z axis) to the surface.
• The object bounding box and world matrix are written to the asset meatadata on file save.
• This is done through a PreSaveFn callback set for object assets via a new AssetTypeInfo struct. This struct may become more important once we support non-ID assets in future.
• For local ID assets, the data should be accessed directly, so the file doesn't have to be saved to get the bounding box updated.

Note: Object assets have to be (re-)saved with this commit or later for the bounding box to appear.

Also:

• The "placement plane" gizmo is scaled to the bounding box size too.
• Added proper gizmo group custom property support to C (was already supported in Python, or in C with some hacking).
• The object preview image is disabled while dragging objects over the 3D view.
• Change the "Add Named Object" drag into string to "Add Object". The "Named" part is kinda useless info and relates to an implementation detail.

TODO

• Allow snapping to both positive and negative normal direction. Currently a ground plane with -Z up will place the object the wrong way around, for example.
• Don't use this for object types with no real geometry, e.g. cameras and lamps.
• Don't draw the bounding box with full edges, just the corners. Makes it more clear that this is just the bounding box.