gst: add some gdb python macros
This adds gdb pretty printer for some GStreamer types.
For GstObject pointers the type and name is added, e.g. "0x5555557e4110 [GstDecodeBin|decodebin0]".
For GstMiniObject pointers the object type is added, e.g. "0x7fffe001fc50 [GstBuffer]".
For GstClockTime and GstClockTimeDiff the time is also printed in human readable form, e.g. "150116219955 [+0:02:30.116219955]".
New version for the patch imported in #320 (closed)
- moved the files to libs/gst/helpers/
- added local copies of the glib/gobject helper functions