desktoppackage: suppress onSettingValueChanged error

In AppletConfiguration, a connection is established to the signal onSettingValueChanged of the loaded config page to enable the apply button. If the page does not have such a signal, as is the case for most if not all pages, an error message is printed every time that page is loaded. This error is completely inconsequential (there are separate mechanisms by which these pages control the apply button), so the message is unnecessary and potentially misleading log spam.

This change sets the connection to ignore unknown signals, supressing the error message. This is the same strategy also applied in plasmoidviewer, see 9d29d6e9.