Add Salsa20 cipher.

Restructure SymmetricCipher implementation to allow multiple backends.

src/crypto/SymmetricCipher.h

@@ -20,20 +20,22 @@
 
 #include <QtCore/QByteArray>
 
-class SymmetricCipherPrivate;
+class SymmetricCipherBackend;
 
 class SymmetricCipher
 {
 public:
     enum Algorithm
     {
-        Aes256
+        Aes256,
+        Salsa20
     };
 
     enum Mode
     {
         Cbc,
-        Ecb
+        Ecb,
+        Stream
     };
 
     enum Direction
@@ -53,8 +55,8 @@ public:
     int blockSize() const;
 
 private:
-    SymmetricCipherPrivate* const d_ptr;
-    Q_DECLARE_PRIVATE(SymmetricCipher);
+    Q_DISABLE_COPY(SymmetricCipher)
+    SymmetricCipherBackend* m_backend;
 };
 
 #endif // KEEPASSX_SYMMETRICCIPHER_H