Issue recurring

Currently start/due can only be specified for monthly recurrences. While it is not needed for fixed daily/weekly modes (yearly mode can make use of it on leap years), it should be possible for flexible modes to have either start or due date based on last recurrence close date (currently plugin decides which one to use, based on availability, but defaults to due date). That's why anchor date should be separated from recurrence mode.

Migration of existing records can be as follows:
* monthly_start/due are converted accordingly,
* all others: if start_date due_date available and due_date empty anchor_to_start=true, false anchor_to_start=false, true otherwise