Can't detect if a window belongs to a child process launched from inside mutter

There is no way for the code inside mutter (like a Gnome Shell extension) to determine if a new window belongs to a child process launched by it or not.

Allowing code from inside mutter to create a child process and delegate on it some of its tasks is something very useful. This can be done easily with the g_subprocess and g_subprocess_launcher classes already available in GLib and GObject. Unfortunately, although the child process can be a graphical program, currently it is not possible for the inner code to identify the windows created by the child in a secure manner (this is: being able to ensure that a malicious program won't be able to trick the inner code into thinking it is a child process launched by it).

Under X11 this is not a problem, because any program has full control over their windows, but under Wayland it is a different story: a program can't neither force their window to be kept at the top (like a docker program does) or at the bottom (like a program for desktop icons does), nor hide it from the list of windows. This means that it is not possible for a "classic", non-priviledged program, to fulfill these tasks, and it can be done only from code inside mutter (like a gnome-shell extension).

This is a non desirable situation, because an extension runs in the same main loop than the whole desktop itself, which means that a complex extension can need to do too much work inside the main loop, and freeze the whole desktop for too much time. Also, it is important to note that javascript doesn't have access to fork(), or threads, which means that, at most, all the parallel computing that can do is those available in the _async calls in GLib/GObject.

Also, having to create an extension for any priviledged graphical element is an stopper for a lot of programmers who already know GTK+ but doesn't know Clutter.

Fixed in !754 (merged)