Enabled the Undo option for the operation "Add background Image". This is to treat adding a background image like opening an image. This behavior is useful when a background image is add using 'drag and drop'.
Reviewers: Severin
Reviewed By: Severin
Subscribers: brecht
Tags: #bf_blender
Maniphest Tasks: T47050
Differential Revision: https://developer.blender.org/D1725