Add synchronous versions of the asynchronous APIs in
libmogwai-schedule-client. These are written from scratch, rather than
sharing much code with the asynchronous versions, since basing the sync
versions on the async ones would involve introducing a new GMainContext
(with all the issues of reentrancy which can occur); and basing the
async versions on the sync ones would involve spawning additional
threads just to schedule D-Bus calls in the existing GDBus worker
thread.

This adds very basic test coverage of them, matching the level of test
coverage of the existing async versions.

Signed-off-by: Philip Withnall <withnall@endlessm.com>