Linux x64 beta

Good news! I've managed most of the hard work involved in getting a Linux x64 version built.

Unfortunately, some features are unavailable on Linux at the current point:

• Spout2 only works on windows.
• Transparency is not available yet. Transparent windows are seemingly impossible to create with SFML, which means my only option is to try to modify SFML's XWindow after it's been created and turn it transparent. I've been trying every trick in the book but it does not want to do it. If anyone wants to look at the source code and have a go, be my guest.
• "Always on top" was unreliable and has been removed on Linux. You can use your system's window manager to pin it to the top instead.

Hopefully this is useful to some of you, sorry it couldn't be entirely matching the windows version yet.

I've marked this as a beta release because I only have a Linux VM to test in, so there may be discrepancies between my own testing and what you see on a real machine.

Please put any bugs related to the Linux release in this comment section, not the main bug thread. Thanks!