There were two distinct code paths for spawning, one async when creating a new
terminal from within gnome-terminal, and one sync when creating a new terminal
from arguments received via dbus. Unify them, always using an async aproach.
This ensures that the TerminalScreen is realised (and has had its size set) by
the time the VtePty is created, so that the child process has the correct size
right from the start, instead of receiving a SIGWINCH shortly afterwards.

vte#188