Src. Parse the iCalendar (org-mode) priority pattern

Src. Parse the iCalendar (org-mode) priority pattern

This implements only the parsing in *lib.winnow* of priority patterns:
`(A)`, `(A1)`, `[#A]`, etc. The priority pattern occurse after any
bullets, numbers or priorities: ` 8. 45 % [#A] Text`.

The operation is the similar to the owne with numbers. The difference
is that the iCalendar priority patterns apply only after the other
patterns.

The *Conf* enum is updated with an *ical* field.

Also tests are provided.