Item underline support

[IAMSolaara]

Hello, is there any way to get an underline on any item?
Possibly being able to customize its width/color (like with background borders?)?

[zcag]

It would be nice to have some border setting where we can specify some of top, bottom, left, right to show. Would be handy as a hover indicator

[IAMSolaara]

Yeah, that's how I wanted to use it.
Right now I managed to simulate a similar effect using backgrounds to indicate which workspace is selected.

[zcag]

@FelixKratz if you're okay with this i can try to make a pr (fyi i have no experience with c or the codebase, just trying to pitch in).

I guess there's 2 options:

1. Controlling the visibility of individual borders
2. Controlling the width of individual borders

With 1, we will have to check the visibility wherever we check border_width for calculations, also it's less flexible than 2, so i guess configuring the width individually would be way to go.