Add support to the _NET_WM_PID atom

Add support to the _NET_WM_PID atom which is needed by many windows managers to correctly associate a SDL window to its process and to related .desktop file and icon for the given host.

EWMH (Extended Window Manager Hints) is a set of conventions (basically extending the original ICCCM, or Inter-Client Communication Conventions Manual) on how to make X clients play along nicely especially assisting the window manager (which is just another X client) do its job.

What this particular protocol (_NET_WM_PID) does, is add a property to the client's window containing the process id of the client's process. The rationale is that if the window manager detects that a particular client does not respond, it can use the pid to kill it.