Fix overlay implementation for Lazy widget

[nicksenger]

Fixes #1583

Currently the Lazy widget in iced_lazy is broken because we assume we can get_mut an Rc which is not unique - both the element & widget tree hold references to this data.

Since we now need a mutable reference to self in overlay to support operations, the compiler will prevent us from producing an overlay with a mutable reference to the same element that is being held by the Lazy & its widget tree node. As a workaround, I've temporarily removed the element from the references held on the Lazy & in the widget tree while the Overlay is in use, and restored it during the overlay's drop. Note that this will break if any widget methods from the parent widget are invoked prior to the overlay being dropped. Currently UserInterface::update is written such that this doesn't happen, but I'm not sure if it's necessarily guaranteed by Iced.

I've also added pick_lists to each item in the Lazy example to demonstrate that the overlays are working properly.

[tarkah]

Looks good! Thanks for fixing this. Option ftw. I hate how overly complex things are due to ouroboros but not sure we can get around it w/ these use cases.

Example tested and working on Linux.

[hecrj]

Note that this will break if any widget methods from the parent widget are invoked prior to the overlay being dropped.

Since overlay borrows mutably from the parent widget, this should be impossible because of borrow rules.

[hecrj]

Looks great! Thanks!