When there is no room to include speaker, fallback to title only

It's better to print just the title than nothing at all. This will happen
when the timeslot is so short we can't fit two lines - we'll still end up
truncating the title in almost every case, but at least we have something.
Prefer smaller font size over truncating

Previous installation had the loop in the wrong order, which meant we
started truncating as soon as it did not fit at maximum size. Since we
have a minimum size in the loop, it's better to wait to truncate as long
as we can.
Include title on same line as timestamp for really short sessions

This is typical for example for the "welcome" session. When there is no
room at all for any description of the session, put a truncated version
on the same line as the timestamp (after a dash) to make sure we have
something at all.