Add SetRepresentedFilename for macOS title bar proxy icons

[trypsynth]

Summary

• Adds wxd_Window_SetRepresentedFilename (macOS only), wrapping wxTopLevelWindow::SetRepresentedFilename.
• Adds a set_represented_filename method on the WxWidget trait, following the same pattern as the existing set_accessibility_label/set_accessibility_description methods (macOS only, no-op elsewhere).

Motivation

This is the standard mechanism native macOS document windows use for the title bar proxy icon: Cmd-clicking the window title shows the file's path in a Finder-style breadcrumb, and the icon itself can be dragged to move or copy the file. Apps that open documents from disk (editors, viewers, readers) generally want this, and wxWidgets already exposes it on the Cocoa port, it just was not wrapped yet.

Test plan

• cargo build -p wxdragon succeeds locally.

[AllenDang]

Please resolve the conflicts