Brand new GUI - All Java now!

While this tool did have a GUI before, I have heard that it didn't work under Linux. Therefore I have updated and written a new GUI for this (which thankfully was pretty easy, as the old GUI already just called the underlying tool per command line) entirely in Java Swing. Now the tool and its GUI should work under any operating system capable of running Java. I took the chance to write a more comprehensive GUI, notifying you of any issues that may occur from bad inputs, though if anyone finds anything out of the ordinary, I'd like if you could tell me. 
I hope you will continue to use Sidescroller Tile Set Maker to create great tile sets!