gdb: implement 'gst-dot' and 'gst-print' commands

This adds two custom gdb commands:

'gst-dot' creates dot files that a very close to what GST_DEBUG_BIN_TO_DOT_FILE() produces. Object properties and buffer content (e.g. codec-data in caps) are not available.

'gst-print' produces high-level information about GStreamer objects. This is currently limited to pads for GstElements and events for the pads. The output can look like this:

(gdb) gst-print pad.object.parent
GstMatroskaDemux (matroskademux0) {
    SinkPad (sink, pull) {
    }
    SrcPad (video_0, push) {
      events:
        stream-start:
          stream-id: 0463ccb080d00b8689bf569a435c4ff84f9ff753545318ae2328ea0763fd0bec/001:1274058367
        caps: video/x-theora
          width: 1920
          height: 800
          pixel-aspect-ratio: 1/1
          framerate: 24/1
          streamheader: < 0x5555557c7d30 [GstBuffer], 0x5555557c7e40 [GstBuffer], 0x7fffe00141d0 [GstBuffer] >
        segment: time
          rate: 1
        tag: global
          container-format: Matroska
    }
    SrcPad (audio_0, push) {
      events:
        stream-start:
          stream-id: 0463ccb080d00b8689bf569a435c4ff84f9ff753545318ae2328ea0763fd0bec/002:1551204875
        caps: audio/mpeg
          mpegversion: 4
          framed: true
          stream-format: raw
          codec_data: 0x7fffe0014500 [GstBuffer]
          level: 2
          base-profile: lc
          profile: lc
          channels: 2
          rate: 44100
        segment: time
          rate: 1
        tag: global
          container-format: Matroska
        tag: stream
          audio-codec: MPEG-4 AAC audio
          language-code: en
    }
}