We were using a custom clipboard monitor, due to some old behaviour
in 2004 where not all X servers supported XFIXES, which allows to
monitor global clipboard changes between processes, which are needed in
Nautilus for copy, paste and link operations, and their availability
check.
Since Nautilus was a single process, it was working for most of the
time.

However recently we split the desktop in a different process, so we
were not able to correctly set the GDK action for clipboard, since
we were relying on sharing the same clipboard in the same process.

In order to fix this, this patch reworks the clipboard handling to a
more modern way, using the owner-changed signal present in XFIXES.
This fixes the clipboard changing between the desktop and Nautilus
and interproccess clipboard handling, fixes small corner cases, and
in the way this patch makes the code design and ownership of clipboard
more clearer.

https://bugzilla.gnome.org/show_bug.cgi?id=771046