Fix some ui glitches for QAbstractSpinBox:
- update geometry when buttons are toggled on/off
- calc button size with subControlRect instead hardcoded 20px
- when buttons are not shown, don't add the button size in
sizeFromContents for common and macOS style
Fixes: QTBUG-39713
Fixes: QTBUG-75303
Task-number: QTBUG-67126
Change-Id: Ibf330c76deb16358a481bba6bd429fff6a5d57ae
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>