Integrate KeePass2RandomStream into KeePass2 reader and writer classes.

This hopefully completes support for reading/writing kdbx <= 2.14 files.

Also fix a bug in KeePass2XmlWriter so it sets Protected="True" in the Value
tag instead of Key.

tests/TestKeePass2Reader.h

@@ -28,6 +28,7 @@ private Q_SLOTS:
     void initTestCase();
     void testNonAscii();
     void testCompressed();
+    void testProtectedStrings();
 };
 
 #endif // KEEPASSX_TESTKEEPASS2READER_H