[Improve file explorer & implement Assimp with lighting/material support]

- Reworked file explorer UI for clearer hierarchy and usability
- Actually wired up Assimp properly for model loading
- Added basic lighting support
- Added independent material support per mesh
(oh my gosh this took 4 days to actually get working, let alone not crashing 😭)

include/Shaders/Shader.h

@@ -16,6 +16,7 @@ public:
     void setBool(const std::string &name, bool value) const;
     void setInt(const std::string &name, int value) const;
     void setFloat(const std::string &name, float value) const;
+    void setVec2(const std::string &name, const glm::vec2 &value) const;
     void setVec3(const std::string &name, const glm::vec3 &value) const;
     void setMat4(const std::string &name, const glm::mat4 &mat) const;
 
@@ -25,4 +26,4 @@ private:
     void checkCompileErrors(unsigned int shader, std::string type);
 };
 
-#endif
+#endif