update the default childGeometryChanged behavior

this forwards the call up the hierarchy, the requests a resize on a
collapsing animation frame
update variable names
cleanup
add a dirty child size flag

this ensures that children are resized on the next resize pass when the
container geometry or a child geometry has changed
update and simplify splitter layout invalidation
invalidate the layout when changing splitter orientation
synchronize the splitter sizes when inserting a new panel