Support "free time"

[cori]

From #1 it would be nice to be able to have time blocks that appear in the Nautilus but that can be "overscheduled". Orginial note:

A way to mark an appointment as "free"
I block my calendar with "Focus Time" blocks and i also show my partner's work schedule. It would be great if those could show in the nautilus for visibility but not block task placement
might not be possible - i haven't read any of the code yet...

this chunk of time on my calendar in meant for tasks, so i want tasks to be placed there. I also have calendar entries for family members' schedules that i would use this way, but the most important things are the ones on my calendar

[tombarys]

Thanks! There are many other scenarios where this would be useful. I am preparing some background for prototyping and implementation.