move shaders into .glsl files and add #include feature

src/node_checkbox.h

@@ -20,6 +20,6 @@ public:
     virtual void draw() override;
     virtual void parse_attributes(kAttribute ka, const tinyxml2::XMLAttribute* attr) override;
     void set_icon(const std::string& icon_path);
-    void set_value(bool checked) { this->checked = checked; update_icon(); }
+    void set_value(bool checked, bool trigger_event = false);
     void update_icon();
 };