Add a splash screen

[K-Meech]

Closes #21

Adds a splash screen with rocket animation that plays when the game starts up. This uses David's layout / graphics - I'm sure the animation could be improved, but hopefully this gives a good template to build on.

The position / size of elements is optimised for the surface screen size, but I've made sure everything scales well for 1920 x 1080 too (as this is what most of us are developing on)

[thompson318]

This all works for me.
Logically it may make more sense if there was also a "quit game" button on the splash screen. At the moment you press "log in" to go to the menu screen, then "log out" on the menu screen actually exits the game, whereas I would expect "log out" to take you back to the start screen, from where you can log back in or quit.

[K-Meech]

Thanks @thompson318 - good point about the log in / out buttons. Probably something to run past everyone in the next catchup meeting? Personally, I find it more convenient to be able to quit from the menu screen rather than having to click twice to log out, then quit - but happy to see what David / everyone else thinks 😄

I'll leave it as-is for now, and we can update in a new PR if needed.