Small post-jam UI update

Thanks to everyone who gave feedback on the game jam submission. Since the voting period is now over, I made some small updates. I'm also using this opportunity to try out the itch DevLog feature :). The game is essentially still the same. The changes are:

* add button to show/hide imps once the picture is completed
* add a bit more animation/color change when an imp is found, to make it clearer where it was exactly
* allow the camera to be moved with the arrow keys or WASD when zoomed in (previously the only way to move the camera was by moving the cursor to the edge of the screen)
* fix some camera behavior that would cause the cursor position to change after zooming in, especially when an imp was found