Release candidate for GalaEngine version 0.2.0!

This update has introduced a few, but very useful features to GalaEngine, making it a much better-suited engine for game development!

Features:

• AssetManager class for loading assets from resource (.gres) files, built by GalaMake.
• InputManager class for a rich input system. Using this class, you bind inputs to names. This even allows for multiple inputs per binding! (e.g., binding keyboard and mouse inputs together).
• SoundManager class for simplified sound playback.
• Entities can now draw onto their current layer via a surfaceLayer member, during OnDraw().
• Tilesets now have a flags field (e.g., for collision and damage), with according interfacing via the TileLayer class.
• Added more functionality to the Window class.
• Added more functionality to the Scene class, providing better control over entities and layers.
• ✨Documentation✨ (here)

Changed:

• Some underlying systems have been changed. Refer to commit history.

Improved:

• Multiple bug fixes.
• Proper use of const and references.
• Multiple optimisations.
• Refactoring of some code.