Add documentation to Time, Scene, Input, Engine and Window headers

[Yeffian]

Works on #104

Changes

Added documentation to the following headers:

• Time.h
• Scene.h
• Input.h
• Engine.h
• Window.h

[Yeffian]

*and the Window docs, forgot about that one

[aryanbaburajan]

@YeffyCodeGit Great! Though you seem to have, not documented any of the members? 😅

[Yeffian]

Ohh wait, I totally forgot about the members! Sorry about that, I'll finish it up tomorrow.