Clean up GTK keyboard event diagnostics.

Clean up GTK keyboard event diagnostics.

When I introduced the KEY_EVENT_DIAGNOSTICS system last month in
commit 769600b22, I somehow didn't notice that it sat next to an
existing system of ifdefs labelled KEY_DEBUGGING, which did some
things worse but some things better.

Now I've expanded both of those into a fairly complete system of
diagnostics (keeping the newer name of KEY_EVENT_DIAGNOSTICS), and
made them use debug() rather than printf() so that in situations where
no standard output is available I can still retrieve the diagnostics
from debug.log.