Pass return_type to prompt async begin functions, rather than finish

Pass return_type to prompt async begin functions, rather than finish

Much like g_dbus_connection_call() we now pass our return_type value
when starting the async operation. This unbreaks vala and various
other bindings that make assumptions about the form of xxx_finish()
async calls.

This is an API/API break, but its to the portion of the library
marked as unstable. Only used by seahorse (in jhbuild) and updated
usage there.