gtk: Capture new-style GLib structured logging messages under d-i

GLib 2.50 replaced the non-machine-readable, text-based logging that uses g_log_set_default_handler() with a more structured logging framework inspired by systemd-journal. Each library can either use the structured or unstructured logging framework; unstructured messages are received by structured log handlers, but the opposite is not true.

For the purposes of this debconf frontend, the interesting libraries include GLib and Pango (which use structured logging) and GTK 2 (which does not). As a result, GLib and Pango debug messages did not go to the syslog in d-i; now they do.

Closes: #988589

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This is helpful when trying to debug #987587 by adding more g_debug() to GTK 2 and Pango.