Port to Linux

[nmlgc]

Bundles the following issues, in order:

• #34
• #19
• #13
• #21
• #45
• #40
• #49
• #22
• #15
• #5
• #9
• #57
• Remove any remaining dependencies on Win32 APIs from core game code
• #4
• #41
• #39
• 64-bit builds
• Build the source code for Linux
• Add to the AUR
• Provide a Flatpak