feat: add interface for changing window's z-level

[z4122]

• [x] Tested on all platforms changed
• [x] Compilation warnings were addressed
• [x] cargo fmt has been run on this branch
• [x] cargo doc builds successfully
• [x] Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• [x] Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• [ ] Created or updated an example program if it would help users understand this functionality
• [x] Updated feature matrix, if new features were added or implemented

On macOS, added set_window_level and get_window_level to set and get window's z-level.

[ArturKovacs]

It just occurred to me that maybe there's some merit in exploring a cross platform API for this functionality.

For example, on Windows, there's: SetWindowPos And on X11, there is a group of functions something similar (e.g. XRaiseWindow, XLowerWindow) see: https://tronche.com/gui/x/xlib/window/stacking-order.html

[madsmtm]

set_window_level was added in https://github.com/rust-windowing/winit/pull/2534, querying is tracked in https://github.com/rust-windowing/winit/issues/2602.