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

tests/gui/TestGui.h

@@ -61,6 +61,7 @@ private slots:
     void testDragAndDropEntry();
     void testDragAndDropGroup();
     void testSaveAs();
+    void testSaveBackup();
     void testSave();
     void testDatabaseSettings();
     void testKeePass1Import();