gmessages: Rebase g_log() on g_log_structured()
gmessages: Rebase g_log() on g_log_structured()

Replace the underlying write_string() call which is the ultimate result
of calling g_log() with a call to g_log_structured(). This means that
all g_log() calls will pass through the structured log handling code
path, as long as they are not already modified or dropped by the g_log()
code (fatal masks, aborts, etc.).

In the case that the default structured log writer is in use, this will
result in the same format of log output to stdout or stderr, as
previously happened. If a non-default writer is in use, it handles the
message as it sees fit.

https://bugzilla.gnome.org/show_bug.cgi?id=744456

johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,endlessm/glib,johne53/MB3Glib