Don't include the indicatorWidth twice on Checkable Buttons

QQuickStyleItem calculates the full size from the content size using the
QStyle's sizeFromContent

The QStyle will add the indicator to the contentSize. We should not
include the indicator in the contents otherwise the indicator gets added
twice.

4px of margins is added to match QRadioButton::sizeHint

Change-Id: I791b0a8585843e66a6d8af456d95652496fe94d4
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>