Add OSXOnShouldTerminate and OSXOnWillTerminate quit hooks

[trypsynth]

Summary

• Adds wxd_App_AddMacShouldTerminateHandler and wxd_App_AddMacWillTerminateHandler, following the same multi-handler callback-vector pattern already used for MacOpenFiles, MacReopenApp, MacPrintFiles, and so on.
• OSXOnShouldTerminate is vetoable: if any registered handler returns false, termination is vetoed, matching the semantics of the underlying wxWidgets virtual.
• Adds on_should_terminate and on_will_terminate to the Rust AppEvents trait, following the same shape as the existing on_reopen_app/on_print_files methods (compiles on all platforms, no-op outside macOS).

Motivation

wxApp::OSXOnShouldTerminate/OSXOnWillTerminate are the standard hooks for reacting to application termination on macOS (Cmd-Q, the dock menu's Quit item, system shutdown/log out). An app that needs to confirm a destructive in-progress action before quitting, or do last-chance cleanup, currently has no way to hook into this through wxDragon; only wxEVT_CLOSE_WINDOW on individual frames is available, which does not cover this path. wxWidgets already exposes both virtuals on the Cocoa port, they were just not wired up in the C or Rust binding layers yet.

Test plan

• cargo build -p wxdragon succeeds locally.