feat: Add set_show_hidden_files option for file dialogs

[bbopen]

Summary

Add FileDialog::set_show_hidden_files() and AsyncFileDialog::set_show_hidden_files()
methods to control whether hidden files (dotfiles) are displayed in file picker dialogs.

Motivation

Applications working with dotfiles (.gitignore, .env, config files) need to show hidden files in file dialogs.

This feature is particularly valuable for macOS and Windows apps targeting general audiences. Frameworks like Tauri enable developers to build cross-platform desktop apps for mainstream users who may not be familiar with platform-specific keyboard shortcuts (e.g., Cmd+Shift+. on macOS). Programmatic control over hidden file visibility provides a more accessible and discoverable experience—users shouldn't need to know obscure key combinations to complete a task the application explicitly supports.

On Linux, the standard Ctrl+H shortcut is more widely known among desktop users, making this less critical (see XDG Portal note below).

Platform Support

Platform	Implementation	Notes
macOS	NSOpenPanel.setShowsHiddenFiles	✅ Works
Windows	IFileDialog with FOS_FORCESHOWHIDDEN flag	Needs testing
Linux (GTK3)	gtk_file_chooser_set_show_hidden	✅ Works (requires --features gtk3)
Linux (XDG Portal)	Not supported	Intentional - see below
WASM	Not applicable

Note on XDG Desktop Portal

The XDG Desktop Portal FileChooser API intentionally does not include a "show hidden files"
option. This is by design - the portal's philosophy delegates UI control to the user and
desktop environment, not the sandboxed application.

Users on Linux with the XDG Portal backend (the default) can press Ctrl+H to toggle
hidden file visibility, which is standard UX across Linux desktop environments.

Reference: XDG Desktop Portal FileChooser Documentation

Example

let file = FileDialog::new()
    .set_show_hidden_files(true)
    .pick_file();

Test Plan

• Compiles on macOS
• Manual test on macOS - hidden files visible
• Test on Windows
• Test on Linux GTK3 (cargo run --example show_hidden --no-default-features --features gtk3)