Support KeePass format 3.00 (used by KeePass2 >= 2.15).

Closes #6

Attachments are now stored in a pool under Metadata instead of in entries.
The protected flag of attachments isn't supported anymore.
New metadata attributes: color, historyMaxItems and historyMaxSize.
Dropped metadata attribute: autoEnableVisualHiding.

tests/TestKeePass2Writer.h

@@ -30,6 +30,7 @@ private Q_SLOTS:
     void initTestCase();
     void testBasic();
     void testProtectedAttributes();
+    void testAttachments();
 
 private:
     Database* m_dbOrg;