Added 'Backup Wallet' menu option

- icon from the LGPL Nuvola set (like the tick) - http://www.icon-king.com/projects/nuvola/
- include 'boost/version.hpp' in db.cpp so that the overwrite version of copy can be used
- catch exceptions in BackupWallet (e.g. filesystem_error thrown when trying to overwrite without the overwrite flag set)
- include db.h in walletmodel.cpp for BackupWallet function
- updated doc/assets-attribution.txt and contrib/debian/copyright with copyright info for new icon

src/qt/walletmodel.cpp

@@ -5,6 +5,7 @@
 #include "transactiontablemodel.h"
 
 #include "headers.h"
+#include "db.h" // for BackupWallet
 
 #include <QTimer>
 #include <QSet>
@@ -239,6 +240,11 @@ bool WalletModel::changePassphrase(const SecureString &oldPass, const SecureStri
     return retval;
 }
 
+bool WalletModel::backupWallet(const QString &filename)
+{
+    return BackupWallet(*wallet, filename.toLocal8Bit().data());
+}
+
 // WalletModel::UnlockContext implementation
 WalletModel::UnlockContext WalletModel::requestUnlock()
 {