Add COLOR property serialization from RFC7986.

Not that I'm particularly fan of changing colour per event, but at the moment, if this is done on a client that supports it, like the Nextcloud web interface, syncing with the Nextcloud server with any client that is based on KCalendarCore will loose the information when sending back a coloured event.

This patch is adding colour support for an incidence, following the description in RFC7986, so a sync client based on KCalendarCore would at least preserve this property value when sending back modified event to the server.

I've added a test to ensure that serialisation and deserialisation is keeping this new property (and additionally checking some basic values like dtStart or summary).