Move the Qt Bridge Importer and Exporter into Qt Creator main repository

The Qt Design Bridge consist of two parts:

1. The design tool plugins for PS, Sketch, etc,, which export an intermediate metadata json file and a couple of assets
2. The Qt Bridge Importer which reads the intermediate file, and generates the QML code.

This task is about part 2.

Note: The Design tool plugins which are in their separate git repositories are not affected! (...although the plugins being implemented in JavaScript are open source by design)

The usual pro arguments for handling code in one instead of several repositories apply here, as-well. In addition, a freely available bridge importer will enable contributions like Inkscape/Gimp/Krita/Younameit importers. Having the bridge importer under open source allows these contributors to understand/enhance/fix the importers.