[server] Add a bool SubSurfaceInterface::isSynchronized() const

The mode is not sufficient to determine whether a SubSurface is in
synchronized mode.

Quoting spec:
"Even if a sub-surface is in desynchronized mode, it will behave as in
synchronized mode, if its parent surface behaves as in synchronized mode.
This rule is applied recursively throughout the tree of surfaces.
This means, that one can set a sub-surface into synchronized mode, and
then assume that all its child and grand-child sub-surfaces are
synchronized, too, without explicitly setting them."