Change Application::SystemSettingsChanging to OverrideSystemSettings

The name Application::SystemSettingsChanging implies that the app is
being notified that system settings are being changed. This is not
what the function does - in fact, SystemSettingsChanging overrides
a settings object with user defined settings

Change-Id: Ibbf821ff3c7ec5b38e2e79751058494c749b6428
Reviewed-on: https://gerrit.libreoffice.org/7629
Reviewed-by: Jan Holesovsky <kendy@collabora.com>
Tested-by: Jan Holesovsky <kendy@collabora.com>