Encapsulate away the lgogdownloader-specific backend bits

[ssokolow]

It occurs to me that, given how simple the interaction between this and its backend is, it could work just as well for tools like youtube-dl if I set up a clean separation between the GUI/queue parts and the parts which define available platforms, build lgogdownloader commands, and watch for failure.

While a bit too frivolous to do while more important user comforts are pending, it'd be nice to at least have the option of writing other backend wrappers.