Possibility to work on multiple operations (rename, change category, change account)

New database version (2) :
		fix_cost for categories
		virtual account

src/model/User.cpp

@@ -1,5 +1,5 @@
 /*
-  Copyright 2010 Grégory Soutadé
+  Copyright 2010-2011 Grégory Soutadé
 
   This file is part of KissCount.
 
@@ -23,6 +23,11 @@ User::User(Database* db) : _db(db)
 {}
 
 User::~User()
+{
+    InvalidateOperations();
+}
+
+void User::InvalidateOperations()
 {
     std::map<unsigned int, std::map<unsigned int, std::vector<Operation> >* >::iterator it;
 
@@ -33,6 +38,8 @@ User::~User()
             delete it->second;
 	}
     }
+
+    _operations.clear();
 }
 
 Category User::GetCategory(const wxString& catId)