Add plain/transparent button variant

[zepfietje]

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[danharrin]

I think I'm going to close this one as I'm not entirely sure the use cases that would be covered that aren't already covered by other button variants in our design system.

[zepfietje]

It replaces the icon button as passing just an icon to a plain button looks identical.

The plain button mainly serves as a way to have an action without attracting too much visual attention, e.g. the cancel button in modal dialogs.

[danharrin]

Why would you use a plain button over a gray button for Cancel in a modal for example? I just fail to see the value here when we already have an icon button.