workaround py2app failure

py2app fails with the dreaded 'assert g_type != TYPE_NONE',
hiding the fact that it's gdk-pixbuf that fails to load,
setting DYLD_LIBRARY_PATH doesn't help either.. but this ugly hack does

xpra/client/gtk3/__init__.py

@@ -3,5 +3,11 @@
 # Xpra is released under the terms of the GNU GPL v2, or, at your option, any
 # later version. See the file COPYING for details.
 
-from xpra.gtk_common.gtk_util import init_display_source
-init_display_source()
+try:
+    from xpra.gtk_common.gtk_util import init_display_source
+    init_display_source()
+except:
+    # for some strange reason,
+    # this may fail on MacOS when running py2app
+    # and we don't care then
+    pass