This function will list the symbols in a traceable in a way that is
suitable for showing in a GtkListView or GtkColumnView. For example, that
requires creating copies of the symbols so that duplicates do not cause
hickups with GtkListItemManager.
This relies on begin/end range for the symbols to create something akin to
an interval tree, albeit with GSequence. If performance needs to be
addressed, can probably augment SysprofSymbol for an interval rbtree.
The goal here is to get an object to represent a symbol so that we can
more easily extend things in the future. We'll still use those pointers
as unique in the stackstash in the future.
