Fix or rewrite scraper engine and scrapers

[dag7dev]

Many years have passed since its first release.

Known demozoo bug:

• it always select the first secondary mirror assuming that is from scene.org. This is wrong and should be carefully checked
• demozoo is missing the screenshots, it always downloads only the first one

Known scrape engine bugs:

• sometimes it sets "screenshots" to None: it should set that field to empty list if no screenshot detected
• it doesn't handle very well other extensions than gb, therefore, it may be a good idea to fix this thing, sometimes in the manifest there is "gbc" but the engine has already renamed the file in gb

Other improvements:

• change the general logic to become more flexible (e.g. select best source, include other extensions)
• write a basic test suite
• test from scratch other scrapers, since they may have become buggy due to change in the master scraper

[avivace]

@dag7dev demozoo is also missing the screenshots, it always downloads only the first one

[dag7dev]

@avivace it is bug of the master engine I think

[avivace]

I believe the "more screenshots" link is simply never navigated, only the first screenshot appearing in the main page is considered.

See: https://github.com/gbdev/database/blob/master/scrapers/py_importers/demozoo.py#L186