Better selective download from launcher.

Dear EPIC please allow to download engine without any source and without other platform libraries so basically Editor would be smaller and targeted to people who just test materials, blueprints, graphics. Currently when selecting other platforms to OFF like Android, OSX, HTML5 and so on but still there are tons of source files and libraries included. I hope in 4.18 options to disable more engine parts effectively and to exclude plugins will be possible.

Bumping it once more.