Add advanced recurring dates

[fpunktk]

Recurring dates can be specified as "every odd thursday" or "every second thursday in the month". But these can collide in some circumstances. So there is a need for recurring dates like "every second even thursday in the month" as a monthly date that does not collide with "every odd thursday".
The downside of this feature is that it seems to be not supported by other calender-software and thus might not be portable.