New Simsu release

I took another small break from my current projects and made some improvements to Simsu. Upon request, I added a row of number entry buttons so that it would be easier to play on a touchscreen device. Inspired by this change, I modified the interface to mimic KSudoku’s behavior: left-clicking a cell toggles the values, and right-clicking a cell toggles the notes. I also added the option to highlight all instances of a number. Last but not least, I added undo/redo support*. Enjoy!

*Because of this addition, old saves will no longer work. You should finish whatever game you are playing before you upgrade.