Recur: Migrated recurrence from Date to ISO8601d

- Changed signature of recur functions, which needed a main.h change, which led
  to removing '#include <Date.h>' from main.h, which necessitated adding the
  sam include to various other source files.  Temporarily.
- Added 'ISO8601d ISO8601d::operator+ (time_t)', because of differences from the
  original Date object.

src/ISO8601.h

@@ -95,6 +95,7 @@ public:
   bool sameWeek   (const ISO8601d&) const;
   bool sameMonth  (const ISO8601d&) const;
   bool sameYear   (const ISO8601d&) const;
+  ISO8601d operator+  (time_t);
   ISO8601d operator+  (const int);
   ISO8601d operator-  (const int);
   ISO8601d& operator+= (const int);