Fix label fallback styles.

When either 'label_font_face', 'label_font_point', 'label_color'
or 'label_hilited_color' is not defined in the configuration
file, these styles should fallback to 'font_face', 'font_point',
'text_color' and 'hilited_candidate_text_color' instead of
Squirrel built-in defaults.