Round corners
There is no native way to set a corner-radius for components / rectangle / anything at all. The best results can be achieved by using a MaskGroup. Keep in mind that MaskGroups can't use 9-patch-images. For some unknown reason (to me) using a Rectangle-Node inside a MaskGroup will only work if you set the opacity of the Rectangle to < 1.
To add a corner-radius simply add a black png with rounded corners to your MaskGroup.
<!-- any component -->
<MaskGroup
id="mask"
maskSize="[400, 225]"
maskOffset="[0,0]"
maskUri="pkg:/images/$$RES$$/yourMaskImage.png"
>
<Poster
id="cover"
width="400"
height="225"
/>
</MaskGroup>