#96 - handle database import/export with backwards compatibility

src/de/deadlocker8/budgetmaster/logic/utils/FileHelper.java

...ocker8/budgetmaster/ui/controller/SettingsController.java

...adlocker8/budgetmasterserver/logic/database/Database.java

...r8/budgetmasterserver/logic/database/DatabaseHandler.java

...8/budgetmasterserver/logic/database/DatabaseImporter.java

...budgetmasterserver/logic/database/DatabaseTagHandler.java

...ocker8/budgetmasterserver/logic/database/OldDatabase.java

...de/deadlocker8/budgetmasterserver/server/SparkServer.java

...r8/budgetmasterserver/server/database/DatabaseImport.java

...aster/tests/server/database/DatabaseImportExportTest.java