Add rounding to grouped apps

[cmihail]

Description

Currently Hyprland allows adding rounding to applications inside decorations, e.g.:

decoration {
   rounding = 10
   ......
}

But this rounding does not apply to the groupbar:

It will be great to add a new rounding setting in groupbar with which to achieve a similar rounding/feel for both applications and the groupbar.

Note: I would like to implement this feature probably somewhere at the end of next month when I will have more time.

[vaxerski]

#2415