[scrollbar] Add theming for rounded corners

[J0hannes101]

This PR adds a new rounded-corners option in the scrollbar{ } category for theming.

Example:

scrollbar {
    padding: 0px 0px 0px 10px;  
    rounded-corners: true;
    handle-width: 6px ;
    handle-color: red;
}

[DaveDavenport]

Thanks for the patch.
I had on my todo list, to turn the handle as a subwidget, so it would inherit all theme theming options of a normal 'widget' (including the rounded corners).

Maybe this solution is easier and sufficient.
Would ' rounded-corners' better be 'handle-rounded-corners'? so it matches with the other properties?

[J0hannes101]

So i should rename it handle-rounded-corners for now?

[DaveDavenport]

It would make it more clear its on the handle, and not the widget itself (that is 'border-radius' where you can specify it for each corner)

[J0hannes101]

@DaveDavenport is everything good for merging, or did i miss something?

[DaveDavenport]

No, just have been very busy.