Position lock

Add an option for locking position of objects. Would be useful to prevent accidentally moving an object when trying to edit properties.

Start by locating the editor entry point that handles object selection and dragging; the issue does not name a file or test. Trace how object properties are edited and how movement is initiated, then define the locked state and verify that locked objects cannot be moved while their properties remain editable.

Written by the indexing model from the issue text.