Introduce SurfaceInterface::childSubSurfacesChanged

Currently, the SurfaceInterface class has a signal that is emitted whenever any sub-surfaces in its sub-tree changes in a way that requires a repaint. It is emitted when a sub-surface is resized, moved, or damaged, etc. There is no way for the compositor to track changes in the stacking order of sub-surfaces efficiently.

This change replaces the subTreeChanged signal with a less noisy signal that's emitted whenever the list of direct child sub-surfaces changes.

The new signal is needed by kwin!656 (merged)