Implement 'Save Database Backup' option

Add an option in the 'Database' menu to save a backup of the current database.

Add unit test for saving database copy

* Open a test database, mark it as modified, and save a copy
* Fail if the copy is not a valid database
* Fail if the original database is saved
* Fail if the original database is no longer marked as modified

src/gui/DatabaseWidget.h

@@ -158,6 +158,7 @@ public slots:
     bool lock();
     bool save();
     bool saveAs();
+    bool saveBackup();
 
     void replaceDatabase(QSharedPointer<Database> db);
     void createEntry();