Issue #77 Move mixin classes to separate files

By separating them from unrelated code, their dependents
can use them without depending on unrelated code, which
in turn helps to reduce cyclic dependencies.

src/blackmisc/comparefunctions.h

@@ -11,12 +11,10 @@
 #ifndef BLACKMISC_COMPAREFUNCTIONS_H
 #define BLACKMISC_COMPAREFUNCTIONS_H
 
+#include <QFlags>
 #include <QDateTime>
 #include <type_traits>
 
-template<typename Enum>
-class QFlags;
-
 namespace BlackMisc
 {
     namespace Compare