LibGUI: Add and use Window::center_on_screen()

Various applications were using the same slightly verbose code to center
themselves on the screen/desktop:

Gfx::IntRect window_rect { 0, 0, width, height };
window_rect.center_within(GUI::Desktop::the().rect());
window->set_rect(window_rect);

Which now becomes:

window->resize(width, height);
window->center_on_screen();

Libraries/LibGUI/Window.h

@@ -123,6 +123,8 @@ public:
     void resize(int width, int height) { resize({ width, height }); }
     void resize(const Gfx::IntSize& size) { set_rect({ position(), size }); }
 
+    void center_on_screen();
+
     virtual void event(Core::Event&) override;
 
     bool is_visible() const;