winit-win32: Fix Window focus after showing hidden maximized window

[rhysd]

• Tested on all platforms changed
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• 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

Fixes #4586

Previously winit could call ShowWindow(SW_MAXIMIZE) while the window was still hidden. It could disturb Win32’s focus state. The fix delays the call until the window is actually shown avoiding calling it twice.

[rhysd]

CI failed inside winit-x11, which should not be related to this PR's change.

error: iterating on a map's values
    --> winit-x11/src/event_processor.rs:1760:31
     |
1760 |                 for window in self.target.windows.borrow().iter().filter_map(|(_, w)| w.upgrade()) {
     |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `self.target.windows.borrow().values().filter_map(|w| w.upgrade())`
     |
     = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.96.0/index.html#iter_kv_map
     = note: `-D clippy::iter-kv-map` implied by `-D warnings`
     = help: to override `-D warnings` add `#[allow(clippy::iter_kv_map)]`