feat: add slider radius option

[Xurdejl]

closes #340

[Keith94]

A value of 2 looks pretty much perfect. More than 2 starts to look oddly distorted, and more than 4 looks increasingly broken.

You think we could cap the value between 0-2 or always use a 2 radius (not configurable)?

[Samillion]

closes #340

Thank you very much. I deeply apologize for the longest delay ever. I had family issues that obligated to focus on it all the time.

You think we could cap the value between 0-2 or always use a 2 radius (not configurable)?

Indeed. Though maybe a note of it would be enough? I'm not sure to be honest.

[Samillion]

Alternatively, we could make it a yes/no option.

For example:

# yes = 2
# no = 0
slider_rounded_corners=yes

Another way:

# rounded_corners = 2
# sharp_corners = 0
slider_style=rounded_corners

[Keith94]

slider_rounded_corners=yes

Yeah, +1 for this and making it the default IMO.