Automatic downloading of assets

[iProgramMC]

Possibly could fetch assets from a contemporary version of Minecraft Java Edition from Mojang's official CDN.

There is still the problem of downloading mobile specific assets such as the blurred background or the touch controls.

Alternatively we can depend on the Internet Archive, and if the required packages aren't there anymore, we could always let the people load them manually in the game.

Asset initialization should be quick (take less than 10 seconds more than the time to download), and should save assets so that they can be used instead of redownloading again.

[Un1q32]

This should prolly be retired in favor of #296 and #276, clasicube textures become the default and people can install a resource pack with the mojang textures if they want.

[BrentDaMage]

Definitely