Factorize some code in GridAccount

Set default font for everybody
Fix a bug in DateDelegate (bad month)
Fix a bug in GridAccount : Tab added for sub operations when saving
Fix a bug in GridAccount : credit was considered as a debit

src/model/User.cpp

@@ -97,7 +97,7 @@ int User::GetCategoryId(const QString& catName)
 
 const QFont User::GetCategoryFont(int catId)
 {
-    QFont f;
+    DEFAULT_FONT(f);
     Category cat;
 
     for (unsigned int i=0; i<_categories.size(); i++)