Native Linux build added

Good news, Linux users! The game now has a native build for Linux. But there's a caveat. Any functionality involving the clipboard (copy and paste) had to be disabled in the Linux version because the Windows version relies on convenient clipboard access provided by System.Windows.Forms, which can't be used in a native Linux build—and there is no equivalent available for me to use that I know of on Linux. That means the somewhat rudimentary puzzle sharing that the clipboard allowed can't be done with the native Linux build, nor can the feature where you paste external image data into the puzzle editor be used.

Another interesting thing to note is that the native Linux build takes up about half as much space as the Windows build. Presumably DirectX MonoGame projects simply have higher space requirements than their OpenGL counterparts, though it's also possible it could have something to do with Windows vs. Linux.

MacOS users, if there are any who are interested in Nonozle, are out of luck, unfortunately. I don't have any Apple computers and I'd need one to be able to properly build and test the game for MacOS.