添加方法：返回ImageBtnGroup中处于被选中状态Button的index
Merge remote-tracking branch 'openwayne/libgdx-ui/master'
Merge remote-tracking branch 'openwayne/libgdx-ui/master'
Merge pull request #2 from Teris-W/master

添加方法：返回ImageBtnGroup中处于被选中状态Button的index