feat: Native GTK4 modal dialog windows for PushModalAsync

[Redth]

Summary

PushModalAsync now presents pages as native GTK4 modal windows (Gtk.Window with SetModal/SetTransientFor) by default, replacing the previous inline widget-swapping behavior.

New API: GtkPage attached properties

Aligned with the macOS MacOSPage API:

Property	Type	Default	Effect
ModalPresentationStyle	GtkModalPresentationStyle	Dialog	Dialog (native window) vs Inline (legacy)
ModalSizesToContent	bool	false	Measures MAUI content and fits dialog to it
ModalWidth / ModalHeight	double	-1	Explicit size (-1 = match parent window)
ModalMinWidth / ModalMinHeight	double	-1	Minimum size constraints

Usage

// Default — full-size native GTK4 dialog:
await Navigation.PushModalAsync(new MyPage());

// Custom size:
GtkPage.SetModalWidth(page, 400);
GtkPage.SetModalHeight(page, 300);
await Navigation.PushModalAsync(page);

// Sizes to content:
GtkPage.SetModalSizesToContent(page, true);
GtkPage.SetModalMinWidth(page, 250);
await Navigation.PushModalAsync(page);

// Inline (legacy behavior):
GtkPage.SetModalPresentationStyle(page, GtkModalPresentationStyle.Inline);
await Navigation.PushModalAsync(page);

Changes

• GtkPage.cs — New attached property class with enum + sizing properties
• WindowHandler.cs — OnModalPushed/OnModalPopped create native dialog windows; ComputeDialogSize handles sizing logic
• ModalDemoPage.cs — Sample page demonstrating all modal options
• README.md / Checklist — Updated docs

Demo

The sample app includes a "🪟 Modal Pages" entry in the sidebar with buttons for each presentation style.