Overhaul AbstractOutput

Summary:
There are several issues with code of AbstractOutput class:

(a) Some methods are documented, and some are not. In general, we tend
    to document all public methods in KWin core. It looks like a very
    minor issue, but there are methods that have very ambiguous return
    value. One such method is geometry(). It's not obvious whether the
    returned geometry is in device independent pixels or not;

(b) There's a mix of methods defined in the cpp file and in the header.
    This is not very good because reading such code becomes a bit harder
    if you don't use any fancy IDE;

(c) Missing Q_DISABLE_COPY, etc.

This change addresses these issues, so the code is a bit more readable
and easier to work with.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: broulik, cfeck, davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D21874