Implement startup ID support.

This provides a basic gtk-shell protocol implementation on top of
aura-shell protocol. Startup IDs from gtk-shell are forwarded to
the aura shell.

_NET_STARTUP_ID property is also forwarded to aura-shell when using
X11 forwarding.

Bug: 828147
Change-Id: Ic52fe446337608cf0318af4c121f5acdbf37a8c6
Reviewed-on: https://chromium-review.googlesource.com/992315
Tested-by: David Reveman <reveman@chromium.org>
Reviewed-by: Jeffrey Kardatzke <jkardatzke@google.com>
4 files changed