A Gtk.TextChildAnchor is a spot in the buffer where child widgets can
be “anchored” (inserted inline, as if they were characters). The anchor
can have multiple widgets anchored, to allow for multiple views.
Determines whether a child anchor has been deleted from
the buffer. Keep in mind that the child anchor will be
unreferenced when removed from the buffer, so you need to
hold your own reference (with GObject.Object.ref()) if you plan
to use this function — otherwise all deleted child anchors
will also be finalized.