Add _NET_WM_PID atom to Linux X11 window

[lindexi]

What does the pull request do?

Append the _NET_WM_PID atom to Linux X11 window.

I learn from gtk3:

// https://gitlab.gnome.org/GNOME/gtk/blob/3.24.1/gdk/x11/gdkwindow-x11.c#L935-944
  if (!gdk_running_in_sandbox ())
    {
      /* if sandboxed, we're likely in a pid namespace and would only confuse the wm with this */
      pid_t pid = getpid ();
      XChangeProperty (xdisplay, xid,
                       gdk_x11_get_xatom_by_name_for_display (x11_screen->display, "_NET_WM_PID"),
                       XA_CARDINAL, 32,
                       PropModeReplace,
                       (guchar *)&pid, 1);
    }

And I learn from qt:

// https://github.com/qt/qtbase/blob/44876fb45e702c2554fca98ed19af57feb1c0511/src/plugins/platforms/xcb/qxcbwindow.cpp#L434-L445
    quint32 pid = getpid();
    xcb_change_property(xcb_connection(), XCB_PROP_MODE_REPLACE, m_window,
                        atom(QXcbAtom::Atom_NET_WM_PID), XCB_ATOM_CARDINAL, 32,
                        1, &pid);

    const QByteArray clientMachine = QSysInfo::machineHostName().toLocal8Bit();
    if (!clientMachine.isEmpty()) {
        xcb_change_property(xcb_connection(), XCB_PROP_MODE_REPLACE, m_window,
                            atom(QXcbAtom::AtomWM_CLIENT_MACHINE), XCB_ATOM_STRING, 8,
                            clientMachine.size(), clientMachine.constData());
    }

// https://github.com/qt/qtbase/blob/44876fb45e702c2554fca98ed19af57feb1c0511/src/corelib/global/qsysinfo.cpp#L948-L957
QString QSysInfo::machineHostName()
{
    // the hostname can change, so we can't cache it
#if defined(Q_OS_LINUX)
    // gethostname(3) on Linux just calls uname(2), so do it ourselves
    // and avoid a memcpy
    struct utsname u;
    if (uname(&u) == 0)
        return QString::fromLocal8Bit(u.nodename);
    return QString();
}

What is the current behavior?

Miss the _NET_WM_PID atom in Avalonia Linux x11 window.

What is the updated/expected behavior with this PR?

We can find the _NET_WM_PID atom in Avalonia Linux x11 window. And we get the the Process Id from Avalonia Linux x11 window by _NET_WM_PID atom.

How was the solution implemented (if it's not obvious)?

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

Fixes #17444

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0053207-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0053460-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[lindexi]

 Host names are limited to MAXHOSTNAMELEN (from <sys/param.h>) characters
  including null-termination, currently 256.

https://man.netbsd.org/gethostname.3