qt gui: default to QT_QPA_PLATFORM=xcb for AppImage

The wayland plugin would require at least debian 12 (or ubuntu 22.04) at runtime.

see https://github.com/spesmilo/electrum/pull/9189#pullrequestreview-2309983943 :

> I've now tried running the appimage on debian 10 (oldoldstable), and am getting an error with wayland.
>
> ```
>   4.16 | D | util.profiler | Plugins.__init__ 0.0422 sec
>   4.16 | I | daemon.Daemon | launching GUI: qt
>   4.76 | I | gui.qt.ElectrumGui | Qt GUI starting up... Qt=6.7.1, PyQt=6.7.1
> /tmp/.mount_electrFlGFOt/usr/bin/python3: symbol lookup error: /tmp/.mount_electrFlGFOt/usr/lib/python3.11/site-packages/PyQt6/Qt6/plugins/platforms/../../lib/libQt6WaylandClient.so.6: undefined symbol: wl_proxy_marshal_flags
> ```
>
> If I explicitly specify `QT_QPA_PLATFORM=xcb`, it starts and works as expected. But it picks wayland by default.
> I found https://bugreports.qt.io/browse/QTBUG-114635 and it looks like even debian 11 might be affected.

electrum/gui/qt/__init__.py

@@ -51,6 +51,14 @@ try:
 except ImportError as e:
     pass  # failure is ok; it is an optional dependency.
 
+if sys.platform == "linux" and os.environ.get("APPIMAGE"):
+    # For AppImage, we default to xcb qt backend, for better support of older system.
+    # qt6 normally defaults to QT_QPA_PLATFORM=wayland instead of QT_QPA_PLATFORM=xcb.
+    # However, the wayland QPA plugin requires libwayland-client0>=1.19, which is too new
+    # for debian 11 or ubuntu 20.04. So instead, we default to the X11 integration (and not wayland).
+    # see https://bugreports.qt.io/browse/QTBUG-114635
+    os.environ.setdefault("QT_QPA_PLATFORM", "xcb")
+
 from electrum.i18n import _, set_language
 from electrum.plugin import run_hook
 from electrum.util import (UserCancelled, profiler, send_exception_to_crash_reporter,