add app.focus function

src/window/Window.hpp

@@ -36,6 +36,8 @@ public:
     virtual void setMode(WindowMode mode) = 0;
     virtual WindowMode getMode() const = 0;
 
+    virtual void focus() = 0;
+
     virtual void setTitle(const std::string& title) = 0;
     virtual void setIcon(const ImageData* image) = 0;
 

src/window/detail/GLFWWindow.cpp

@@ -468,6 +468,10 @@ public:
         return mode;
     }
 
+    void focus() override {
+        glfwFocusWindow(window);
+    }
+
     void setTitle(const std::string& title) override {
         glfwSetWindowTitle(window, title.c_str());
     }