• Instead of holding references to the cursor state of a Window during cursor loading, an event is sent.
• The event can now properly handle loading a cursor for multiple Windows.
• Reduce the amount of Arc upgrades to an absolute minimum.
• Drop references to ImageBitmap, HTMLElementCanvas and ImageBitmapRenderingContext as early as possible to free up resources.
• Drop object URLs even in case of an error.
• Always check if the cursor actually changed before updating it through Window::set_cursor().

Addresses #3306.