skins2: improve layout management

skins2: improve layout management

Be more stringent on what it means to be tightly-coupled for two layouts.

Note that directly resizing a playout should not be allowed. Only the skins2
windows manager should be authorized to do it, since other issues are at stake
(anchors, min/max constraints, ....). Yet, this new isTightlyCoupledWith() now
ensures that two layouts are truly identical from the windows manager's
perspective, and thus avoids possible and undesirable side effects.