gstinfo: Add new API for getting debug log lines
If you're using a custom log handler, you had to reverse-engineer the
debug log format and create your own format function. Now, you can
gst_debug_log_get_line() and it will return a string (without
ANSI escape color codes) representation instead.
This is useful in situations when you need to log the ordinary
gst_debug log to a resource that can't be opened as a