Update Component Checkbox

Note:

z-checkbox-cnt have not yet changed to z-checkbox-content in

norm.less :

.z-label, .z-radio-cnt, .z-checkbox-cnt, .z-loading {
	font-family: @baseContentFontFamily;
	font-size: @fontSizeMedium;
	font-weight: normal;
}
Merge branch '7.0' of https://github.com/zkoss/zk into update-checkbox
Change z-checkbox-cnt to z-checkbox-content
Fix IE9 gradient