The following changes were done to (hopefully) achieve backwards
compatibility while allowing themes to change the size of the indicator.

(1) Deprecate the property.
(2) Change the default value of the property to 0. If it is not 0,
    use the property's value for the indicator size. This should make
    all programs that actually set it keep the size they set it to.
(3) If set to other values than 0, use min-width/min-height of the
    check/radio node to size the indicator. This allows themes to change
    the size.
(4) Fall back to the previous default size of 16px. This way themes that
    do not set the size keep the same behavior.