Use libsoup rather than GIO for HTTP downloads

To get direct access to HTTP protocol features not exposed by GIO, such as headers (and thus implement #641809), Steadyflow will need to forsake the GIO abstraction specifically for HTTP. Currently HTTP support in GIO pulls libsoup through gvfs-backends, which, despite a cleaner API, introduces overhead for D-Bus IPC compared to just using libsoup directly.

One option I've been pondering is tearing out SoupInputStream, basically a GIO adapter for asyncronous libsoup - exactly what we need - from gvfs-backends and compiling its C code alongside Steadyflow's Vala code. In the long term, we could consider porting it to Vala.