Implement subsurface.place_below() for parents
From the second commit:
Flatten the subsurface actor tree, making all surface actors children of the window actor. Save the subsurface state in a GNode tree in MetaWaylandSurface, where each surface holds two nodes, one branch, which can be the tree root or be attached to a parent surfaces branch, and a leaf, which is used to save the position relative to child branch nodes. Each time a surface is added or reordered in the tree, unparent all surface actors from the window actor, traverse all leaves of the tree and readd the corresponding surface actors back to the window actor.
This does not take surface offset into account any more, see !684 (comment 555420) for context why we shouldn't.
Implements #440 (closed)