[Feature] Transparent color setting for "color".

[ensan-hcl]

Currently, "normal", "special", and "selected" are allowed as colors of keys.

I found that some iOS keyboard have "transparent" keys. They are typically used in number pads and they are relatively unimportant keys.

Considering current color declarations are based on semantics, adding "unimportant" key color which is treated as transparent key will make sense.

[ensan-hcl]

Implemented in develop branch. This feature will be released in CustardKit 1.1