In preparation for the addition of the GSListener abstract class for X11
and Wayland implementations. This matches file naming, incidentally.

This is the right format, previous commits have already changed some of
it.

In preparation for using WleGtkSocket on Wayland.

In preparation for using WleGtkPlug on Wayland.

The precautions taken in this commit to avoid X11 errors are not always
sufficient. For example, it's possible to get one of these errors on
resuming from suspend when a notification appears at the same time (not
just any notification, apparently; in my case it's from nm-applet).
However, I can't reproduce the problem with openbox, only with xfwm4,
and apparently only with /general/unredirect_overlays enabled.

So user can extend or override the flags provided there by defining the
DISTCHECK_CONFIGURE_FLAGS variable.

First step in distinguishing X11 and Wayland, the rest will come in
next commits.

* Remove unused project files
* Remove dead code/checks/flags
* Fix X11 specific headers

The build actually failed in the !HAVE_SIGACTION case, so it seems safe
not to check for the presence of this function.

It's not xfce4-screensaver's job as long as xfce4-power-manager exists.
The DPMS should only be configurable in one place, which simplifies life
for both the user and the developer.

This search for the best visual doesn't seem very useful, and the GDK
functions that would make this code independent of the windowing
environment have been deprecated, so let's just remove it and use the
system default visual instead.

This reverts commit 75243250.

Real fix coming in next commits.

Fixes: #161

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