Make the new login button stuff more automatic

By default it'll look at your environment to try to figure out which
display manager is used in order to start a new session.  We first try
the org.freedesktop.DisplayManager dbus interface, and if that fails,
inspect XDG_SESSION_DESKTOP to try to figure out which display manager
is running.

The user can also still specify the correct display manager, or a custom
command.

Cargo.lock

@@ -318,6 +318,7 @@ dependencies = [
  "bscreensaver-util",
  "glib",
  "gtk",
+ "log",
 ]
 
 [[package]]
@@ -347,9 +348,11 @@ dependencies = [
  "humantime",
  "lazy_static",
  "libc",
+ "shell-words",
  "toml",
  "xcb",
  "xdg",
+ "zbus",
 ]
 
 [[package]]