Sprite related improvements

[lesleyrs]

• always hide cursor since the sprite cursor is enabled, added 1 version of all sprites, cutting sprites, and hide cursor #50
• Add cutting (Ctrl-x) to sprite editor added 1 version of all sprites, cutting sprites, and hide cursor #50
• Clean up sprite sheet Remove random scribbles from sprite sheet #53
• Ctrl-s moves down the currently selected sprite by 1 row: Use arrow keys for sprite shifting (instead of WASD) #55
• what happened to .ppm files being generated on save? - Intentional 3e84646
• palette swap tool sprites instead of 2 versions change icons to unselected variant #68
• is cargo clean just required for it to pick up replaced sprite_sheet.txt files? Replaced files not being noticed by Cargo #57
• save/read to and from clipboard when using ctrl-x/c/v What still needs to happen for clipboard to be supported? #48 and ingame access

The following are not that important to do right away and don't apply to me that much. Maybe it's better to just put them in comments so we can finish this issue sometime? I think some of these are already commented as well. Or track these in a separate issue with other sprite editor features.

- [ ] change cursor sprite based on what is being hovered
- [ ] undo/redo does not work for pasting sprites, only for the sprite editor itself.
- [ ] right click to grab palette
- [ ] Ability to move the selected sprite using keys: `q` `w` OR `a` `z` with or without `shift`
- [ ] Ability to change the selected palette using keys: `1` and `2` Also fun idea with this: allow shift modifier to move 2 palettes at a time and it has more features than the original pico8

I didn't expect to be thinking about this many things when just drawing a few sprites but there you go. I don't want to track the actual sprite editor features here as it's already excessive.