Fix most of critical issues reported by Sonar (GTK client) (#2309)

* (C++) Macros should not be used to define constants

* (C++) Memory should not be managed manually

* (C++) "void*" should not be used in typedefs, member variables, function parameters or return type

* (C++) When the "Rule-of-Zero" is not applicable, the "Rule-of-Five" should be followed

* (C++) "switch" statements should have "default" clauses

* (C++) "explicit" should be used on single-parameter constructors and conversiosn operators

* (C++) Non-const global variables should not be used

qt/Prefs.h

@@ -16,9 +16,9 @@
 #include <QVariant>
 
 #include <libtransmission/quark.h>
+#include <libtransmission/tr-macros.h>
 
 #include "Filters.h"
-#include "Macros.h"
 
 class QDateTime;