This reverts commit 09853c06.

Incorporate feedback for virtual timeslots

See merge request !57

API consumers must not be tempted to parse ids, so we encode them as
UUIDs to prevent that.

Up until now the id of the first and last program entry was based on the
filtered range. Instead, we want these to be based on the first timeslot
that was scheduled before/after the specified range. This way we ensure
that the ids of these entries are stable as long as the schedule doesn’t
change.

Mapping keys in filter_data are guaranteed to be defined but might be
None for boolean filters. We want to ensure that we actually pass
booleans to generate_program_entries.