This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

For debugging.

Fixes build failure on GLib < 2.32.

New GLib versions don't fail.

Replaces io_thread_call().  This approach is more generic and easier
to use due to std::function.

Fixes deprecation warnings.

GLib 2.16 was released more than 4 years ago.  Let's remove some cruft
from the glib_compat.h header, and avoid new cruft to it.

Ensure that the io.thread variable is set before entering the event
loop.

Same as _timeout_add_seconds(), but this one has millisecond
resolution.

The numeric ID isn't so useful.

Try to eliminate the remaining blocking I/O.