[Idea] Improve direction changing method

Splitted from https://github.com/petitlapin/Li-Ri/issues/13:

```
In gameplay, the direction changing method is confusing. I don't know how the direction changes
when touching the screen, and it takes time to loop among possible directions.
```

Proposed solution:
```
Add a soft keyboard in game, which consists of four direction keys and a pause button.
```
