Code Cleanup

- Migrated the recur.cpp getDueState() function into Task::getDateState(),
  which can assess any date attribute.
- Improved implementation to distinguish between:
    - not due, or not due for rc.due days
    - due after today
    - due later today
    - due earlier today
    - due before today
  This greater precision should address some outstanding issues.

src/main.h

@@ -43,8 +43,6 @@ void handleRecurrence ();
 Date getNextRecurrence (Date&, std::string&);
 bool generateDueDates (Task&, std::vector <Date>&);
 void updateRecurrenceMask (Task&);
-int getDueState (const std::string&);
-int getDueState (const Date&);
 bool nag (Task&);
 
 // rules.cpp