UIButton 设置图片文字位置 -- imageEdgeInsets && titleEdgeInsets

发现一篇讲得有些原理的博客：
UIButton的titleEdgeInsets属性和imageEdgeInsets属性实现图片文字按要求排列
里面还有写好的category方法可以直接使用。

下面是图片右边，文字左边的一个按钮设置方法：

UIButton *rightButton = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 100, 44)];
    [rightButton setTitle:@"这个是一个导航栏按钮" forState:UIControlStateNormal];
    rightButton.titleLabel.font = SYSTEMFONT(14);
    [rightButton setImage:[UIImage imageNamed:@"icon_filter"] forState:UIControlStateNormal];
    [rightButton setTitleEdgeInsets:UIEdgeInsetsMake(0, -rightButton.imageView.bounds.size.width, 0, rightButton.imageView.bounds.size.width)];
    [rightButton setImageEdgeInsets:UIEdgeInsetsMake(0, rightButton.titleLabel.bounds.size.width, 0, -rightButton.titleLabel.bounds.size.width)];
    [rightButton addTarget:self action:@selector(testFunction) forControlEvents:UIControlEventTouchUpInside];
    self.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc] initWithCustomView:rightButton];