Add iced::window::run_with_window_handles for usage e.g. with rfd 0.15+

[flakes]

Based on

• several discussions on Discord and rfd for file dialogs #1002
• the latest update to rfd which took away the auto-parent selection for asynchronous rfd dialogs on MacOS

I decided to give this a chance. There's a few rough edges, specifically:

• naming and placement of the new structure WindowHandles
• there's no way to start another task from within the closure that is passed to run_with_window_handles, so we can't use async code without another intermediate Message
• usage of unwrap()
• removal of run_with_handle (?)

All comments welcome :-)