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

COPYING

@@ -124,66 +124,67 @@ Copyright: 2003-2004, David Vignoni <david@icon-king.com>
 License: LGPL-2.1
 Comment: from Nuvola icon theme
 
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+       share/icons/application/scalable/actions/password-generator.svg
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        share/icons/application/scalable/actions/reports.svg
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+       share/icons/application/scalable/apps/preferences-desktop-icons.svg
+       share/icons/application/scalable/apps/preferences-system-network-sharing.svg
+       share/icons/application/scalable/apps/utilities-terminal.svg
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+       share/icons/application/scalable/mimetypes/application-x-keepassxc.svg
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 Copyright: 2019 Austin Andrews <http://templarian.com/>
 License: SIL OPEN FONT LICENSE Version 1.1
 Comment: Taken from Material Design icon set (https://github.com/templarian/MaterialDesign/)