Implement image asset handling in generator tool

This currently saves a copy of the image with a default name "svg_asset" in the current directory and refer to that. Either we find a way to pipe the actual path/URL used in the SVG through, so that we can refer to this in the QML file as well, or if we decide to keep the duplication, we should add options to the tool for setting the location etc. But we should probably make the URL available to the generator.

It also currently saves non-alpha images as JPG which is a lossy format, so this should probably always be PNG.