EXO: Refactor wl_subcompositor interface

Move the wl_subcompositor interface into its own files. This change only
moves code around, no functional changes.

Bug: 896710
Test: Build and run exo_unittests
Change-Id: I7675d455bb46d356025d1fd6479ac61bf4b92bae
Reviewed-on: https://chromium-review.googlesource.com/c/1427568
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624965}
4 files changed