Removed pygame dependency, allow using system tor and update over tor if the latter is used.

This pull request contains several features:

- I have removed the depency for pygame as this feature is none critical. Instead the option is grayed out and a tooltip appears inviting the user to install pygame if he desires the feature.

- Added an option to use the system wide installation of tor. What this does is set the proper environment variables TOR_SKIP_LAUNCH, TOR_SOCKS_HOST, TOR_SOCKS_PORT.

- When the previous feature is enabled (and the recommended python-txsocksx package is installed) allow the user to do updates (and checks) over Tor. This feature however is only available when using a system install of Tor (as the built-in Tor isn't launched when doing updates).