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.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;