allow editing calendar on existing events

[jonas-haeusler]

these changes have been lying around for some time, finally found the time to finalize them and create this merge request.

in essence, this will just delete the old event and recreate it with a new id in the new calendar. please test thoroughly and let me know if something is unclear.

fixes a bunch of open (duplicated) issues, here a few i found while quickly scanning the backlog: fixes #62 fixes #101 fixes #812 fixes #1266 fixes #1400 fixes #1410

and also fixes #1351 and fixes #1134: the color picker is now hidden if the calendar does not provide any colors.