settings: Fix xsettings handling

settings: Fix xsettings handling

I was somehow under the misconception that we'd get GdkEventSettings
events for all the xsettings at startup. That is not in general true,
so we need to make sure that we check for the xsettings value before
we use them, or derived fields. Update all the private getters to
do so; and fix settings_update_font_values() to cope with font
descriptions that might miss the family or size.