Support for partially rounded Button

[Vinci10]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

#5352 added support for partially rounded rectangles. It allows to create partially rounded buttons as well. It may help users to implement custom button widgets like Material Design: Button groups or Split buttons

Is it possible to construct a solution with the existing API?

It is not possible because Button background is not public

Describe the solution you'd like to see.

I think new public function added to Button e.g. SetCornerRadii(topLeft, topRight, bottomLeft, bottomRight) or new theme constants SizeNameButtonRadiusTopRight, ... should be sufficient

[andydotxyz]

It may help users to implement custom button widgets like Material Design: Button groups or Split buttons

A better solution would be to have the button group / split button widget in Fyne itself - simpler API and based on user-case which is our design preference.

The widgets don't expose design controls typically so adding the missing widgets fits the existing API.