Make AbstractEglBackend a QObject

Summary:
Several of the subclasses are already derived from QObject.

The main reason is that the class should be moved out of KWin core in
order to move the OpenGL scene into a plugin. As Compositor calls into
the AbstractEglBackend to unbind the wayland display this creates a
problem which is easily solved by turning the AbstractEglBackend into a
QObject and connect to the signal emitted by the Compositor.

Test Plan: Compiles

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7669