Add arena sprite handling

[deonix37]

1. Make arena's border to be an actual border instead of a solid square, for the ability to change the transparency.
2. Change Show and Hide to enable and disable the sprite, so it doesn't reset the transparency if we change it.
3. Add a humble set of methods to control arena's inner sprite.

(And do we still need to check for defending state in LuaArenaStatus? Arena is nil outside wave scripts for me).

[RhenaudTheLukark]

I checked the talk we had in the Unitale server about this feature, and I think it's a good idea to bring up what's the end goal here.

There are two different choices we can make here:

• We could either completely cover the arena on the engine side, which forces us to have a sprite for each corner, a sprite for each side, and a sprite for the inner part of the arena,
• Or we could just make the arena hidden easier, which would allow users to handle their arena themselves on the Lua side.

The first one would include an array with nine different sprites in it, while the other would look more into making the arena hidden at all times, with no in-engine call to the function that would show the arena once more.

I personally don't know which solution is the best one to pick, and so if you have an input on how you think it should be done, I wouldn't mind it one bit. All I know is that the current system is a little lackluster.

Also, I'm sure we can keep your new color functions, since that's something which has been requested, and is a fine addition overall.