checkbox's clickable area is needed to be expanded in some cases

Expand the clickable area of qcheckbox,making it more easy to use

If you need the checkbox to be checked from everywhere over it when you insert　it into other widget such as qlistwidget,qtablewidget,
qtreeview,etc, you can set state to true, which can make it more humanlized and easy to use in a variety of different requirements
scenarios.

The following example is a usage scenario for QCheckbox, which needs to expand the clickable area.And the image qcheckbox.png below shows the range of clickable areas before and after the change