Bug: Removing root object does not clear use_root_object flag; instance_offset row incorrectly disabled

[Weisl]

Problem

Two related UI bugs in the collection list:

1. Clicking "Remove Root Object" sets collection.root_object = None but leaves use_root_object = True, so the collection still behaves as if a root object is set.
2. The entire row containing both the root_object picker and instance_offset field was disabled when use_root_object was True, making the root_object prop itself un-clickable.

Expected behaviour

1. Removing the root object should also set use_root_object = False.
2. Only the instance_offset field should be disabled when use_root_object is True (the root object picker must remain interactive).