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]".
Fixes#320
