[app] Use X11-specific API under guards
[app] Use X11-specific API under guards

We should be using X11-specific API (either GDK-X11 or direct
Xlib calls) only under the GDK_WINDOWING_X11 define.

The only X11-specific code is the slowly_and_stupidly_obtain_timestamp()
function we ported over from Nautilus. This function only has sense when
dealing with startup notification on X11, so if we are not building for
X11 we can safely use g_get_current_time() (we are building a fake startup
notification id anyway).

This commit should hopefully make Libunique a tad more portable; we still
need a replacement IPC mechanism for non-Unix platforms, but I hope in
D-Bus being ported first. Other platforms startup notification mechanisms
should be implemented in GTK+ already.

Distrotech/libunique,Distrotech/libunique,Distrotech/libunique,GNOME/unique,GNOME/unique,GNOME/unique