Add Accept header to web-session.capnp and change proxy/bridge to use it

Seems like this would match any param name ending in a 'q'. Also, is whitespace allowed around the =?

I think a safer parsing approach here is:
- Split on all semicolons, not just the first. Event though "q" is the only parameter name defined, the intent of the grammar is that there could be multiple.
- The first item in the resulting list is the mime type. (Trim it.)
- Each remaining item is a name=value. Split on first '=' to separate them.
- If name.trim() == "q", then set qvalue to parseFloat(value.trim()).