Add Birthdays from Contacts as a separate Calendar

[devvv4ever]

as discussed here: #685

I'm opening this as an issue since this comes up in the tickets on a regular basis, too.

My thoughts on this:

1. I think a DAVx5-app-wide calendar that gathers birthdays from all accounts/addressbooks would be the easiest to be used for the end user. However it would be best in the "tools" menu then.
2. Alternatively we could also add one calendar in the CalDAV tab of each account. Benefit: Users have it shown right away and would not need so search it in the tools and it is more there where it belongs (= Contacts and Calendars). They have then the possibility to switch on contact birthdays per-account. But: if they have multiple accounts they would have multiple Birthday calendars in the calendars list from various sources. A bit more messy probably - but offers more flexibility also.
3. The biggest felxibility (and the biggest mess of calendars) would be if we add it as an option inside an address book (maybe in the details view).

If we want to be very clever (most work of course) and if we choose the 3rd option, we could think of making this an option in the DAVx5 settings ("Activate a calendar that contains selected birthday sources") and make one "global" DAVx5 calendar for all birthdays, which contains all the birthdays people activate in their address books.