chore: Fix compiler warnings

[OliW07]

Addressing various compiler warnings to clean up the build output.

Changes:

• shell-util: Remove unstable slice_internals feature, add memchr crate dependency
• systray-util: Handle PostMessageW return value with let _ =
• desktop: Remove unused iterator_try_collect feature flag
• widget_factory: Prefix placement with _ for non macos compilation
• path_ext: Add #[allow(dead_code)] for to_relative method (helper function likely useful in future)
• open_meteo_res: Add #[allow(dead_code)] for wind_direction field (feature pending [Feature Request] Add wind direction display to weather provider #279)
• sys_tray: Fix lifetime elision in icon_image return type