libsysprof-gtk: show a list of backtraces on the right

When selecting a callback node, show all a list of all of the traces where
that occurred on the right, so that you can select them individually.

Currently this only shows pid/time for on the right, but we'll add another
pane that will show the symbolized backtrace soon.

src/libsysprof-gtk/sysprof-callgraph-view-private.h

@@ -37,6 +37,7 @@ struct _SysprofCallgraphView
   GtkColumnView *callers_column_view;
   GtkColumnView *descendants_column_view;
   GtkColumnView *functions_column_view;
+  GtkColumnView *traceables_column_view;
   GtkCustomSorter *descendants_name_sorter;
   GtkCustomSorter *functions_name_sorter;
   GtkScrolledWindow *scrolled_window;