gdb: add 'gst-pipeline-tree' command

gdb: add 'gst-pipeline-tree' command

It shows a simple tree of all elements in pipeline.
As with gst-dot, the toplevel bin is found from any element of the
pipeline:

(gdb) gst-pipeline-tree bsink
playbin
  inputselector1
  inputselector0
  uridecodebin0
    queue2-0
    decodebin0
      avdec_aac0
      aacparse0
      vaapidecodebin0
        vaapipostproc0
        capsfilter1
        vaapi-queue
        vaapidecode0
      capsfilter0
      h264parse0
      multiqueue0
      matroskademux0
      typefind
    typefindelement0
    source
  playsink
    abin
      aconv
        resample
        conv
        identity
      aqueue
      pulsesink0
    vbin
      vconv
        scale
        conv
        identity
      vqueue
      vaapisink0
    vdbin
      deinterlace
      vdconv
    audiotee
    streamsynchronizer0