EXO: Refactored wl_shell interface

Move wl_shell interface  out of server.cc
This CL only moves code around.

Bug: 896710
Test: Built on ChromeOS
Change-Id: Ic9b4a8d0cb68c30c0836af103c326aee70f66811
Reviewed-on: https://chromium-review.googlesource.com/c/1323969
Commit-Queue: Shawn Gallea <sagallea@google.com>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608644}
4 files changed