Add ability to swap sprites by index

[TDRRmk2]

Would be massively helpful for importing sprite sheets and replacing sprites that way. Right now, you have to import the sprite sheet, then start swapping one sprite at a time with CTRL+[ until you finally get to the same index the sprite you want to replace is in. This is very tedious for basically any amount of sprites and simply being able to immediately swap places with the currently selected sprite and a specified index would speed this up massively.

[kirjavascript]

You can fullscreen the sprites pane and drag them into place faster with the mouse

I'll have a think of what UX I can add to make this easier