tasklist: Add only one "Start New Instance" item for grouped buttons

Rather than having a "Start New Instance" item prepended to each grouped
button's submenu, append the item after the individual instance's
submenu items. Use the last instance's PID to figure out the path to
launch.
2 jobs for !5 with tasklist-start-new-instance in 4 minutes and 7 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Build
passed #2185
build

00:01:35

 
  Distcheck
passed #2186
distcheck

00:02:32