Support for inline data in the URL

This came up from VectorImage not supporting inline data as source instead of just url to actual SVG file. This could be handled by supporting an URL embedding the data, e.g.

"source: "data:image/svg+xml;utf8," + '<?xml..."

This should be a general mechanism that can be used anywhere we load files, presumably some virtual file engine like qrc that is appropriately handled in QML.