Free canvas sizing

Another overhaul from the previous version. You can now use any size canvas you like, but the grid maxes out at 120x40.

New features include:

• Free canvas size
• Preview in the lower-right corner
• Rectangles and circles, filled or empty

Plus bug fixes and quality of life stuff. There are a few others I'd like to add when I have time. I tried implementing unicode, but it caused more problems than it was worth. You can still use alt-codes.

Oh and I made new icons. Nothing groundbreaking, but I think they turned out pretty nice. And a logo based on the MSPaint logo.