Partial implementation of scenes

- Moved main game content into a scene
- Added `grid_x` and `grid_y` functions to `Grid` entity
- Made red `Transition` entity for map transitions (later)
- Created isOccupied method for dynamic object checks
Refactoring of isOccupied function

We can't use `this` (since it refers to the window instance) but we can
use `self`.
Added a nice loading screen.

And moved backgrounds into the scene. Loading scene generates in reverse
colours (black bg, white text).
Added on-walk animation.
Resolved #1. Remove transitions. We don't need them.