Refactor GUI codebase

[ghost]

The objective of this commit is to make the GUI code mostly workable.

• Uniformize coding style.
• Use OOP constructors.
• Encapsulate functionalities.
• Give more descriptive names to variables.
• Enforce 80-column rule.
• Remove noisy coments.
• Reap dead code.
• Localize function references that run every loop.
• Reuse pixel images.
• Fix #35.