Simple Property Inspector

After messing around with ImGui, YUGEN now has its basic interface! The Property window allows user to browse scene objects (currently not support nested objects), and freely modify the parameters of different components.

I've finally decided to leave model format as it is now. Model shall be loaded in its original extension with ASSIMP library in the level editor, but optimized by having our own model format in the game release. I'd like to finally achieve something like Unity does - one directory containing all resources/libraries needed and one executable. Clean, and highly-portable.

Noted that all the paths are relative to the executable file. Users need to specify their resource directory on their first use. Also, you can always reset to the default docking layout by clicking the top-left button.

There're already couple issues with the GUI, especially with the Scene window, but they're so trivial that I probably won't care to fix them now. Instead, there will be skybox and rendering stuffs coming in the next update.