Add love.window.focus (resolves #1911).

Calling the function brings the window into the foreground and makes it the focused window for keyboard input.

src/modules/window/sdl/Window.cpp

@@ -1182,6 +1182,15 @@ void Window::restore()
 	}
 }
 
+void Window::focus()
+{
+	if (window != nullptr)
+	{
+		SDL_RaiseWindow(window);
+		updateSettings(settings, true);
+	}
+}
+
 bool Window::isMaximized() const
 {
 	return window != nullptr && (SDL_GetWindowFlags(window) & SDL_WINDOW_MAXIMIZED);

src/modules/window/sdl/Window.h

@@ -87,6 +87,7 @@ public:
 	void minimize() override;
 	void maximize() override;
 	void restore() override;
+	void focus() override;
 
 	bool isMaximized() const override;
 	bool isMinimized() const override;