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」
Improve label fallback colors.

Improve default label fallback foreground colors by
blending candidate text foreground color and
background color using the method described in
'https://github.com/lotem/weasel/blob/master/RimeWithWeasel/RimeWithWeasel.cpp#L452'.
Merge pull request #13 from lembacon/master

Support more per-color-scheme settings, Add a new color scheme ‘clean_white’ & Improve label fallback colors