Quality of life stuff

Hey, no bigger update this time but still some stuff that should be nice. Here's a list of changes:

- Alpha in color picker:

You can now set the alpha value of colors in the colorpicker, meaning you can have for example a layer of transparent clouds. Alpha is enabled for all colors so you could even make the planet itself transparent if you want.

- Pixel margins in spritesheet:

Sometimes spritesheets don't always work in every engine. This is because the frames are to close together in the image, and pixels of different tiles can "bleed" over to different frames. To counteract this you can now set a margin in the spritesheet menu that will put an empty border around every frame.

- Faster png exporting

I updated the method of png exporting making it easier to download big images. This doesn't really effect gif exporting much though.

And unfortunately I have to take the MacOS download offline for now. Since I don't own a mac I can't test it. Also apple does not allow you to build MacOS apps from a non-apple machine, and the method I used previously no longer works.

Hope you like update though, cheers!