Separate UI Logic from Rendering Implementation

[hectr]

This pull request refactors the framework to separate the UI primitives, diffing, etc. from the rendering implementation. The goal is to decouple the core logic of the framework from the specific rendering backend, making it easier to support alternative backends.

Key changes:

• Renderer is a protocol implemented by TerminalRenderer;
• There is a new InputHandler protocol implemented by TerminalInputHandler;
• Renderer and InputHandler are injected to Application;
• Window and Control have a common abstract parent class: LayoutObject;
• LayerDrawing, Cell, CellAttributes, EscapeSequence, and ArrowKeyParser are now implementation details of the terminal renderer backend.

Many more ACLs are public now, as Renderer and InputHandler implementations might need visibility of all the controls and other supporting types.

Only tested on macOS 14.6.1 - Xcode 15.4 (15F31d)

EDIT: Tested also on Linux with official Docker image. On Windows it won't build (see #4).