Replace standard fields with a state system

[SomeSourceCode]

Description

The current implementation stores values like x, y, layoutX, layoutY, visible, etc. as standard fields. This makes it impossible to listen for changes.

Instead, those values should be stored inside observable states. This enables listening for changes and reacting accordingly, such as firing updates, triggering re-renders, etc.

Some states should be read-only, such as x, y, since they're calculated based on other values.

Motivation

This is motivated by #19. AnvilGui should support an observable text input field.

Expected Code

// getters and setter persist:
node.setLayoutX(x);
node.getLayoutX();

// states can be accessed and observed:
node.layoutXState().observe((oldValue, newValue) ->{
    // ...
});

// read-only states can't be set:
node.xState().set(x); // won't compile