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.