[ozone/wayland] Factor "data device manager" out of WaylandConnection

CL adds a WaylandDataDeviceManager class and moves that handle
wl_data_device_manager logic out of WaylandConnection into it.

It brings no functionality change, but makes it easier to be extended
to support primary selection (middle-click to paste).

BUG=578890

Change-Id: I8c04035bf4d3e9de5a0c2cac5c83d4552912cc0d
Reviewed-on: https://chromium-review.googlesource.com/1035943
Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#554910}
5 files changed