看 manual：
checkable : bool
This property holds whether the button is checkable.
By default, the button is not checkable.
Access functions:
bool isCheckable () const
void setCheckable ( bool )
See also checked.
checked : bool
This property holds whether the button is checked.
Only checkable buttons can be checked. By default, the button is unchecked.
Access functions:
bool isChecked () const
void setChecked ( bool )
See also checkable.
