QuickShow: Animate animated images :^)

With a little help (read: copy & paste) from ImageWidget, QuickShow will
now cycle through the frames of animated images - enjoy the cat GIFs!

Future improvement: cache decoded images like LibWeb's ImageResource to
waste less CPU - the same applies to LibGUI though, maybe we can put
something shared in LibGfx.

Closes #5837.

Userland/Libraries/LibGUI/ImageWidget.cpp

@@ -71,6 +71,7 @@ void ImageWidget::set_auto_resize(bool value)
         set_fixed_size(m_bitmap->size());
 }
 
+// Same as QSWidget::animate(), you probably want to keep any changes in sync
 void ImageWidget::animate()
 {
     m_current_frame_index = (m_current_frame_index + 1) % m_image_decoder->frame_count();