Adjust unsafe blocks for the Window impl

[peamaeq]

In this function you use the unsafe keyword for almost the entrie function body. However, I found that only 5 functions are real unsafe operations (see the list below).

We need to mark unsafe operations more precisely using unsafe keyword. Keeping unsafe blocks small can bring many benefits. For example, when mistakes happen, we can locate any errors related to memory safety within an unsafe block. This is the balance between Safe and Unsafe Rust. The separation is designed to make using Safe Rust as ergonomic as possible, but requires extra effort and care when writing Unsafe Rust.
Real unsafe operation list:

1. the init_window()/run_loop()/DeleteMenu()/get_win_os_error()/Shell_NotifyIconW() function(these are unsafe functions)

Hope this PR can help you.
Best regards.
References
https://doc.rust-lang.org/nomicon/safe-unsafe-meaning.html
https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html