Use Qt xcb platform by default

CURA-12054

It is now a fact that Cura doesn't work properly with native Qt/Wayland integration. However, on recent Ubuntu versions Wayland is now used by default. So we now force using the xcb platform unless otherwise specified, which should have no impact on older platforms but work better on new ones.

packaging/AppImage/AppRun

@@ -9,6 +9,7 @@ export QML2_IMPORT_PATH="$scriptdir/qt/qml"
 export QT_QPA_FONTDIR=/usr/share/fonts
 export QT_QPA_PLATFORMTHEME=xdgdesktopportal
 export QT_XKB_CONFIG_ROOT=/usr/share/X11/xkb
+export QT_QPA_PLATFORM=xcb
 
 # Use the openssl.cnf packaged in the AppImage
 export OPENSSL_CONF="$scriptdir/openssl.cnf"