Add a "move forward" and "move backward" layout action

[ftessier]

It would be great if, in addition to the "move to front" and "move to back" actions for object depth, there were a "move forward" and "move backward" action, that move the object by a single step in depth. For slides with many objects, this would curb complicated selection and depth adjustment gymnastics! Moreover, this would be consistent with most other object editors out there. The "move forward" could be represented the current ^ caret symbol, and "move to front" with a double-caret (and similarly for backward motion).