xwayland: Check X11 clients prior to terminate Xwayland
Currently, mutter checks for the presence of X11 windows to decide whether or not Xwayland can be terminated, when Xwayland is started on demand.
Unfortunately, not all X11 clients will map a window all the time, an X11 client may keep the X11 connection opened after closing all its windows. In that case, we may terminate Xwayland while there are some X11 client connected still, and terminating Xwayland will also kill those X11 clients.
To avoid that issue, check the X11 clients actually connected using the XRes extension. The XRes extension provides the PID of the (local) X11 clients connected to the Xserver, so we need to match that against the actual executable names, and compare with a list of known executables that we can safely ignore, such as ibus-x11 or gsd-xsettings.
We also check against our own executable name, considering that the X11 window manager is also an X11 client connected to the Xserver.
Also, XRes returning the PID of local clients only is not a problem considering that Xwayland does not listen to remote connections. However, if the user spawns a client remotely on another system using ssh tunneling (ssh -X), only clients which actually map a window will be accounted for.
Closes: #1537 (closed)