Fixed bugs

Was passing the cumulative time instead of the tick time to each object.
Also fixed such that all objects accelerate due to all others, which
wasn't working before.
Added time counter to top-centre of screen. Shows `hrs`, `days`, after
start of simulation.
Added text and a solar system

Text added to all items.
Added proper timing

Timing now works. Define an FPS with `fps 20` and a value for simulated
seconds per visual second with `t 3600` (one hour per second).