Add autocomplete styles

[merlindiavova]

Add style properties specific to autocomplete.

style.completion_background = { common.color "#ede7db" }
style.completion_text = { common.color "#000000" }
style.completion_type = { common.color "#4a3a8a" }
style.completion_text_active = { common.color "#8a290f" }
style.completion_infobar_divider = { common.color "#a60000" }
style.completion_infobar_background = { common.color "#fbf7f0" }
style.completion_text_accent = { common.color "#000000" }

Might be useful for others.

[adamharrison]

Hey, sorry it took so long to review this.

So we had a bit discussion in PR committee meeting number 8 about this; and we came to the conclusion that while we do wanna be able to add more colors like this, especially for plugins. Currenlty the mechanism to do this is just generally to put things in config.plugins.autocomplete.completiong_background or something.

Doing it in style like this, and having fallbacks causes problems with themes that aren't the default theme, because you'll never fallback (i.e., you'll always use the default theme values for these), due to the way the style stuff is inherited.

We'll put our a PR to address this with a new system of some kind, sometime soon; we can adjust this PR at that point to use it.