Improve Integration test so that we can have easily accessible difference image in all platform in case of an error

After whacking head against a wall in Windows when there is no readily accessible tool to calculate image difference between PNGs, and trying with Python(s) resulted in unforeseen installation problems, it seems best to make it in Qt for ALL the platform. This also means that the difference image is available everywhere instantly the test is run and no further tinkering is needed.