libsysprof-analyze: add callgraph flags for thread-ids

This allows you to set a flag to show the thread id of what was recorded.

Use this to help disassociate similar threads in a process to figure out
which thread is consuming a majority of the Total time of that process.

src/libsysprof-gtk/sysprof-symbol-label.c

@@ -48,6 +48,7 @@ static const char *kind_classes[] = {
   NULL,
   "root",
   "process",
+  "thread",
   "context-switch",
   "user",
   "kernel",