Add repositories in separate files in sources.list.d

[Ryan Thompson]

The repository-adding feature of Ubuntu Tweak should add repositories in separate list files in /etc/apt/sources.list.d. This is a much simpler way of doing things, because each repository is in a separate file. Ubuntu Tweak should also offer an option to migrate any non-standard repositories (or at least the supported repos) out of sources.list and into separate files.