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 non-monthly modes, 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 Conversion of existing records can be as follows:
* monthly_start/due are converted accordingly,
* all others: if due_date available anchor_to_start=false, true otherwise