Make M-left and M-right affect only the item, not its children
This behavior is now parallel to the treatment of outline nodes.
This commit also introduces another change. When an outline node or a
plain list item is folded by outline and contains hidden children,
M-left/right will refuse to act on this item. You must either open
the tree, or use the subtree commands M-S-left and M-S-right.
Based on a patch by Matti De Craene, but significantly modified after
a discussion involving Bernt Hansen and others.