Distributions do package each backend plugin in a dedicated package,
which means just because KWin got compiled with a specific backend,
doesn't mean it is also available at runtime.
In order to better support this reality this change introduces a
list-backends command line option. Each of the plugins got the json
metadata extended by the KPlugin syntax, so that we have a name and
description to print.
As we already locate all plugins anyway, the additional findPlugins for
the selected backend is changed to search the list of all plugin meta
data.
Each of the backends becomes a plugin. This allows kwin_wayland to load
the requested plugin and kwin itself doesn't need to link all the
libraries needed. E.g. libdrm is no longer linked if running kwin_x11.
Also this allows to create backends for the non-standard EGL platforms
(examples could be raspberrypi or Android devices).
