MetroForm.CancelButton() can't be set to a MetroButton

### Bug explanation

this designer code does not work.
```
            private ReaLTaiizor.Controls.MetroButton btn_cancel;
            this.CancelButton = btn_cancel;
```
Last line gives:

> CS0266	Cannot implicitly convert type 'ReaLTaiizor.Controls.MetroButton' to 'System.Windows.Forms.IButtonControl'. An explicit conversion exists (are you missing a cast?



### Version

3.8.0.7