Refactor Windows platform implementation (#11393)

This aligns the Windows platform implementation with a code style
similar to macOS platform, eliminating most of the `Cell`s and
`Mutex`es. This adjustment facilitates potential future porting to a
multi-threaded implementation if required.

Overall, this PR made the following changes: it segregated all member
variables in `WindowsPlatform` and `WindowsWindow`, grouping together
variables that remain constant throughout the entire app lifecycle,
while placing variables that may change during app runtime into
`RefCell`.

Edit: 
During the code refactoring process, a bug was also fixed.

**Before**: 
Close window when file has changed, nothing happen:


https://github.com/zed-industries/zed/assets/14981363/0bcda7c1-808c-4b36-8953-a3a3365a314e

**After**:
Now `should_close` callback is properly handled


https://github.com/zed-industries/zed/assets/14981363/c8887b72-9a0b-42ad-b9ab-7d0775d843f5


Release Notes:

- N/A

crates/gpui/src/platform/windows.rs

@@ -1,14 +1,18 @@
 mod direct_write;
 mod dispatcher;
 mod display;
+mod events;
 mod platform;
+mod system_settings;
 mod util;
 mod window;
 
 pub(crate) use direct_write::*;
 pub(crate) use dispatcher::*;
 pub(crate) use display::*;
+pub(crate) use events::*;
 pub(crate) use platform::*;
+pub(crate) use system_settings::*;
 pub(crate) use util::*;
 pub(crate) use window::*;