[RoundButton] Rework the contentItem, improve background accuracy
The contentItem now uses more of RoundButton's public API.
The background now looks a bit more like Breeze.
Note: the default size has changed noticeably. The size this patch uses is more correct because it uses padding for the content rather than only depending on the background being large enough. This means devs can now prevent icons and text from spilling out of the rounded edges.