Support more per-color-scheme settings & Add a new color scheme 'clean_white'

# Support more per-color-scheme settings

All currently available settings under __style__ can now be also added to each __color scheme__.

List of __style__ settings that have just become per-color-scheme settings:
* horizontal
* label_font_face
* label_font_point
* label_color
* label_hilited_color (hilited_candidate_label_color)
* font_face
* font_point
* alpha
* corner_radius
* border_height
* border_width
* candidate_format

If any of these settings is presented under each color scheme, then the setting will be interpreted as to override the corresponding global setting with the same name under the __style__ section.

This will make it easier for color scheme creators to predefine the font styles, candidate styles, window styles and etc.

_P.S._ It would be more appropriate to rename __color scheme__
to __theme__ in the future.

# Add a new color scheme clean_white

__简约白／Clean White__, based on 搜狗「简约白」.

![Screen Shot 2013-01-17 at 3 06 16 PM](https://f.cloud.github.com/assets/251435/74046/69cb8cbc-6074-11e2-9f75-c914730405e2.png)