Add colors utilities

[Athozus]

This is a rework of mail colors. It is a prerequisite for #102, as the addition of it will add many colors. So, it has the following changes :

• Remove colors from init.lua
• Add an utility file /util/colors.lua
• Only one global function mail.get_color() which returns a color from mixed base colors. It overpasses the mix if there is only one color given.
• Local function get_base_color(). The inheritor of init.lua. It uses shortcuts of one letter to be simplified and notably for the use of mail.get_color(). Of course, comments to tell what is what.
• Local functions to convert from hex to rgb and vice-versa.
• Local function to mix rgb colors.
• EDIT : and, of course, it replaces all the previous occurrences of mail.colors

An example of use :

• Old : mail.colors.imp_add_sel
• New : mail.get_color("ias"), mail.get_color("sai") ...

You can note that the new utility is independent from order.

NB : I wrote the whole code in less than 1 hour, so the review should be quick =)

[S-S-X]

Didn't yet read all of it but here's what I think about current colors.lua and shortcuts:

I'd suggest either direct math or caching for colors.lua. Also I feel that single letter shortcuts aren't really improvement but instead makes it harder to follow what is actually happening as you have to think about meaning of those codes.

If you like you could implement it with + operator (and others if you like fancy) too so that you can mix colors like colors.header + colors.highlight - colors.blue

[BuckarooBanzay]

Also I feel that single letter shortcuts aren't really improvement but instead makes it harder to follow what is actually happening as you have to think about meaning of those codes.

☝️

The color.lua code is a bit overkill imo but if you think it is needed 🤷

[Niklp09]

imo better code: (should be more performant too)

local colors = {
    ["h"] = "colorstr"
}

function mail.get_color(style)
    return color[style]
end

[Athozus]

Sorry for being late, I reworked the utility using the following syntax :

mail.get_color("header")
mail.get_color({"selected", "important"})

Note that in the utility itself, the table as suggested by @Niklp09 is called generic_colors to not break colors arguments of some functions.

Hope this one will be ok for you 👍