- fixed a few issues in bom exporter:
 * variant counts are now updated correctly
 * better display of variants : variant names default to "default"
 * exporting to Json file (and auto download) actually works
 * refactored bom exporter view a bit : moved the creation of parts collection to bomExporter itself (even here it is not quite its place
but that is for another refac)
 * download url request now works correcly
 * cleaned up the main bomExporter view