Make restack_items() available to all TraceTreeItemOwners

Currently, TTIO:restack_items() is empty and only the
TraceGroup actually implements it. For this reason, decode
traces (and analog traces) are correctly pushing other traces
around when their size changes.
Top-level traces that are not part of a group however aren't
handled properly, simply overlapping when they grow.

This patch makes the code available to all TTIO-derived
classes, allowing all owners (including View itself) to
properly handle this event.