[client] Track all created Outputs and add static get method

Summary:
This change starts to track all Outputs and introduces a static
method to get an Output* for a wl_output* in case the wl_output* is
known to Output.

This is needed for the enter and leave events on wl_surface.

Reviewers: #plasma_on_wayland

Subscribers: plasma-devel

Tags: #plasma_on_wayland

Differential Revision: https://phabricator.kde.org/D2526