fix: uninstall does not remove the droplet

kit_monthly_calendar
updated DE language file
refactoring of the SQL usage in the class.backend.php
refactoring of the SQL usage in the class.backen.php
refactoring of the SQL usage in the class.frontend.php
refactoring of the SQL usage in the droplet.extension.php
updated and adapted initialize.php
updated and adapted install.php
updated precheck.php
removed class.event.php
updated CHANGELOG
updated upgrade.php