Function to modify the labels on a given thread by id.
Other thread:
gm_delete_thread(),
gm_threads(),
gm_thread(),
gm_trash_thread(),
gm_untrash_thread()
if (FALSE) { # \dontrun{
gm_modify_thread(12345, add_labels = "label_1")
gm_modify_thread(12345, remove_labels = "label_1")
# add and remove at the same time
gm_modify_thread(12345, add_labels = "label_2", remove_labels = "label_1")
} # }