Get rid of Rectangle over Qt Quick Layouts if it is transparent

Figma bridge exports auto-layout as Qt Quick Layouts.

This is nice, but since it seems that Figma bridge automatically treats Frame as Rectangle, it just generates redundant Rectangle over Qt Quick Layouts.

Take this RowLayout for example. To me, the Rectangle in line 69 is totally unnecessary and should've been gotten rid of if the Figma exporter finds out that this is transparent.