Only hide preview image while dragging over 3D View

Make the preview hide and unhide depending on if the user drags over the
3D View. In there the preview is too much visual noice, together with
the bounding box and snapping plane we draw there.

Properly set rotation when dropping

Didn't respect the rotation mode before.

Adapt to changes in master

Merge branch 'master' into asset-browser-snap-dragging

Merge branch 'master' into asset-browser-snap-dragging

Merge branch 'master' into asset-browser-snap-dragging

Adjustments to drag info drawing

* Change the "Add Named Object" drag into string to "Add Object". The "Named"
part is kinda useless info and relates to an implementation detail.
* Draw drag info closer to the cursor. Looked quite off before.

Merge branch 'master' into asset-browser-snap-dragging

Fix merge error after last merge

Merge branch 'master' into asset-browser-snap-dragging

Merge branch 'master' into asset-browser-snap-dragging

Merge remote-tracking branch 'origin/master' into asset-browser-snap-dragging

Snap based on the bounding box when dropping, not origin

This makes the dropping use the object's bounding box for snapping, so
that it matches the bounding box we draw while dragging.

Not that this may have to be optional in future, there may be valid
cases to use the origin instead of the bounding box for snapping.

Merge remote-tracking branch 'origin/master' into asset-browser-snap-dragging

Merge remote-tracking branch 'origin/master' into asset-browser-snap-dragging

Merge remote-tracking branch 'origin/master' into asset-browser-snap-dragging

Fix crash when dragging emtpy into the 3D View

Merge branch 'master' into asset-browser-snap-dragging

Show a bounding box for object assets while dragging

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

Basic idea is:
* Store the bounding box (and related info) in the asset metadata.
* The bounding box has to be updated and written to custom properties of the
asset metadata.
* There's a new `AssetTypeInfo` struct, to set asset type dependent callbacks
or data (in future). ID types can conveniently define asset-type info as part
of their `IDTypeInfo` definition.
* The asset-type info can contain a custom callback to be executed before the
asset is saved. This is how the object can write its custom asset metadata.
* The dropbox can also register a callback to set properties for its
gizmo-group now. Used to copy
* The "placement plane" gizmo is scaled to the bounding box size too.
* 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.

Also:
* Added proper gizmo group custom property support to C (was already supported
in Python, or in C with some hacking).
* Store the basis matrix (world matrix without parent or constraint transforms)
in the custom asset metadata. Used by the placement gizmo to scale the
bounding box to match the result visually when dropping.
* The preview image is disabled while dragging objects over the 3D view.
* The bounding box always snaps with its lower edge (Z axis) to the surface.
Snapping with other axes can be added still.
* However, actually dropping the object still uses the origin, so the drop
position may not match what the bounding box indicated. This could be changed
to also use the bounding box instead. Or the drawn bounding box should be
translated to respect the origin position.

Merge remote-tracking branch 'origin/master' into asset-browser-snap-dragging