include "radius" information for a rectangle, also when generating assetData

In the Figma Quick Controls project, we use Figma templates to create a Quick Controls style. And for most of the controls states, we tell the exporter to produce images rather than QML code. The latter is done by using a hack where we assign a drop shadow effect to the Rectangle, to force the exporter to generate an image.

The problem is that the exporter will then not include key information about the Rectangle, like the radius, since it's now exporting an asset instead. But the radius is still needed for the stylegenerator tool that later reads the qtbridge json file and figures out how the background images can be stretched (using 9 patch technology).

So we need that the json file still includes the radius of the rectangle, even when generating an image/asset from it. This can e.g be done by adding the radius to the "metadata" section of a child, or add a new "metadata" section to the "assetData" section, where we key information about the image can be stored.