Allow 5 minute duration goals

[tijlleenders]

Is your feature request related to a problem? Please describe.
It would be very cool if I can not only schedule 1h blocks but also 5 minute tasks on the calendar.

Describe the solution you'd like
Make the step generator and step placer handle this correctly - without impacting performance negatively (I'm guessing <50ms for the default goals so that total recalculate remains <100ms).

Describe alternatives you've considered
Keep working with 1h blocks for now. Not interesting as that seems to be largely sorted out.

Additional context
We still need a test case before this can start.
My feeling is they would 'pool' together once the schedule gets tight. So for a very busy schedule these micro-tasks would bundle in the half-hour 'cracks' in my schedule so I can then complete them all in a row.
We probably need to add some extra logic to the scheduler to keep the 'regular' 1h-based blocks aligned to the whole hours; you don't want a whole day of appointments that all start at :05 just because you started your day with a 5min phone call...

[tijlleenders]

Don't start work on this till we remap to the new API in #386

[tijlleenders]

Don't start thinking about this until all test cases pass in the new scheduler approach from #411

[thinkrapido]

Related #18 (comment)