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.cpp

@@ -1694,6 +1694,12 @@ bool ISO8601d::sameYear (const ISO8601d& rhs) const
   return this->year () == rhs.year ();
 }
 
+////////////////////////////////////////////////////////////////////////////////
+ISO8601d ISO8601d::operator+ (time_t delta)
+{
+  return ISO8601d (_date + delta);
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 ISO8601d ISO8601d::operator+ (const int delta)
 {