GUIScrollbar: Add styling

[kilbith]

This PR adds different styling to the scrollbar element.

This is still WIP, lots of styling properties are missing.

style[scrbar_inv;bgimg=scrbar_bg.png;up_arrow_bgimg=default_mese_crystal.png;down_arrow_bgimg=default_diamond.png;thumb_bgimg=trump.png]

@v-rob Thoughts?

[v-rob]

Looks pretty good. From a quick look at the code, it appears that it support state styling, right?

I still wonder if it wouldn't be better for styling parts of elements to be done with something analogous to CSS pseudo elements selectors. It would be nice if scrollbars were actually made of buttons visible to formspecs since it would make styling easier, but it's not much use wishing for the impossible.

(And, those are some... interesting scrollbars. I trust you have better actual styling for i3 :P)

[appgurueu]

Looks nice. I think the black arrows are unneeded if we can provide arbitrary arrow images.

[v-rob]

I would assume the black arrows would be replaced by fgimg, not bgimg.

[AFCMS]

👍 I am really awaiting this feature!

[Wuzzy2]

This needs documentation in lua_api.txt.

[rubenwardy]

This is still WIP, lots of styling properties are missing.

It's fine for this to be merged with only partial styling support, as long as it supports the bare minimum: hover/down states and images for the thumbs and track. More can be added later, this is long overdue

[AFCMS]

@kilbith Why did you close this pr?? It looked to cool!

[kilbith]

Shit project isn't worth my time and my talent until @hecktest comes back.

[Wuzzy2]

May I take over this PR?

[appgurueu]

I could really use the bgimg (for a color picker). For now I'll have to put the gradient below the scrollbar.

[kilbith]

@Wuzzy2 Feel free to take it over, yes. I intent to work on inventory list styling.