macOS: add dynamic accepts_first_mouse callback

[tilladam]

Summary

• Add ApplicationHandlerExtMacOS::accepts_first_mouse callback that receives the window ID and click position (PhysicalPosition<f64>), enabling per-click decisions about whether to accept first mouse on inactive windows.
• Add EventHandler::handle_with_result for synchronous, re-entrant-safe handler dispatch that returns Option<R>. Refactor existing handle on top of it to eliminate duplication.
• When the handler is unavailable (not set or re-entrant), the static accepts_first_mouse value from WindowAttributes is used as a fallback.

Motivated by slint-ui/slint#10451 — applications need per-click decisions to follow the macOS convention of accepting first mouse for low-risk actions (selection, scrolling) but rejecting it for buttons and destructive actions.

Implementation notes

• acceptsFirstMouse: must return bool synchronously, so the handler is called via handle_with_result which returns None on re-entrancy instead of panicking.
• The NSEvent parameter is Option<&NSEvent> per Apple's API contract (can be nil); nil falls back to the static default.
• Coordinates use the same conversion as existing mouse event handlers (convertPoint_fromView + scale factor), with isFlipped providing top-left origin.

Test plan

• cargo build -p winit-core / winit-appkit / winit — compiles
• cargo test -p winit-core — passes
• cargo test -p winit-common --features event-handler — 5 new tests pass:
  • handle_with_result returns value (normal path)
  • handle_with_result returns None when handler not set
  • handle_with_result returns None on re-entrancy via handle
  • handle_with_result returns None on re-entrancy via itself
  • handle still panics on re-entrancy (regression test)
• cargo clippy -p winit-appkit — no warnings
• cargo +nightly fmt -- --check — clean