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/feedback.cpp

@@ -33,6 +33,7 @@
 #include <stdlib.h>
 #include <inttypes.h>
 #include <Context.h>
+#include <Date.h>
 #include <ISO8601.h>
 #include <main.h>
 #include <text.h>