[omemo] Improve QR code

Set error correction level to "L", which reduces the amount of transferred data.
Error correction level "L" seems to be sufficient for QR codes scanned from computer screens.

omemo/gtk/key.py

@@ -188,12 +188,11 @@ class KeyDialog(Gtk.Dialog):
 
         import qrcode
         qr = qrcode.QRCode(version=None,
-                           error_correction=2,
-                           box_size=4,
+                           error_correction=qrcode.constants.ERROR_CORRECT_L,
+                           box_size=6,
                            border=4)
         qr.add_data(ver_string)
         qr.make(fit=True)
-        qr.make()
 
         fill_color = 'black'
         back_color = 'white'