Handling of appointments (imported from Google) / Expired tasks

[Konzertheld]

• Appointments are always due on the day they are -> always have a deadline
• but it makes no sense to move them forward, if they are over, they are over
• so instead, show them as expired on the current day, but do not modify them in the database
• when clicked on the "expired" tag mark mark as done on the original date
• otherwise allow to mark as "done" today (as we can do for all tasks, see erledigte Aufgaben automatisch auf den aktuellen Tag schieben #9)
• this should only happen for appointments, not affect expired tasks
• we can tell them apart: appointments have a deadline time, too (actually currently they don't because of a bug that the time is not imported)
• expired tasks should be added to "today" regularly, automatically moving tasks without deadline and with unexpired deadlines if possible
• we should propably make an automated "make room" function that allows for moving tasks without and optionally with deadline using some priorization
• could be displayed as a low horizontal bar between the task entering field and the tasks view, showing events horizontally (but see also Section to show everything from all Google Calendars for today #34)