wayland: Track already existing subsurfaces in SubSurfaceMonitor
When the monitor is created, the surface may already have child sub-surfaces. The childSubSurfaceAdded signal won't be emitted for them, we need to handle this case explicitly.
BUG: 433511 FIXED-IN: 5.21.2
Note that this bug can't be reproduced with !656 (merged) because it already handles this case. \o/