Fix broken translation template (pot) generation. (#7425)
The superBrl braille display driver includes escape sequences which produce non-ASCII characters; e.g. "\xff". Due to a gettext bug, this causes xgettext to fail even though these aren't in translatable strings. See https://github.com/nvaccess/nvda/issues/2592#issuecomment-155299911.
1. To work around this, declare encoding as UTF-8 for the superBrl driver.
2. Make AppVeyor build the translation template so that builds fail for problems like this. This way, we learn about such problems long before they get to the translation system.
3. Explicitly exclude source\comInterfaces for scons pot to avoid unknown encoding warnings. These files never contain translatable strings anyway.
