Multiple planets

I added basic support for multiple Planets. Each Planet is simulated individually with its own Energy tracking.
Right now there’s no way to ‘interact’ between Planets, so they’re basically individual games.
For now I simply added 4 Planets with different sizes and seeds:

vec![
    Planet::new(1337, Size { w: 30000, h: 30000 }),
    Planet::new(1338, Size { w: 5000, h: 5000 }),
    Planet::new(1339, Size { w: 2000, h: 10000 }),
    Planet::new(1340, Size { w: 9000, h: 40000 }),
]

I also added buttons to the Hotbar to switch to the next or previous Planet.