Async and cancellable dialogs

[Descolada]

Addresses #838 — adds an asynchronous, cancellable way to show message boxes, common dialogs, and modal dialogs across platforms.

This started as just needing async/cancellable MessageBox.Show on GTK, but grew into a fairly complete implementation covering message boxes, the common dialogs (color/font/file/folder), and custom Dialogs.

API

• MessageBox.ShowAsync(...) — async overloads mirroring MessageBox.Show, each taking an optional CancellationToken.
• CommonDialog.ShowDialogAsync(parent, CancellationToken) — covers OpenFileDialog, SaveFileDialog, ColorDialog, FontDialog, SelectFolderDialog, AboutDialog, etc.
• Dialog.ShowModalAsync(CancellationToken) / Dialog<T>.ShowModalAsync(CancellationToken).

Semantics:

• With no token (or a non-cancellable one), the dialog is simply shown asynchronously on every platform — the native modal loop keeps the UI responsive while the returned task is awaited.
• With a cancellable token, the platform handler must support cancellation; if it can't, a NotSupportedException is thrown.
• An already-cancelled token short-circuits and never shows the dialog.
• A user-initiated close (which may carry a Dialog<T>.Result) completes the task normally; the task only faults with OperationCanceledException when the token closed it.

Shared orchestration lives in AsyncModalDialog: it posts the blocking ShowDialog onto the UI thread (where the native modal loop pumps) and invokes the handler's cancel callback — while it's showing — to break that loop when the token is signalled.

Windows: WH_CBT hook

The original draft of this PR used a hidden dummy owner form because there was no reliable way to identify the native dialog to dismiss it. That's replaced with a cleaner approach:

Just before the blocking ShowDialog call, a thread-local WH_CBT hook is installed (Win32.ModalDialogHook). It captures the dialog's window handle on first HCBT_ACTIVATE, then unhooks itself. Cancellation posts WM_CLOSE to that exact handle — so the right dialog is dismissed unambiguously even when several are open, with no host/owner window and no flicker. Handlers compose this via the small Win32.CancellableModalDialog helper.

This covers, on both WinForms and WPF: MessageBox, ColorDialog, FontDialog, OpenFileDialog/SaveFileDialog, and SelectFolderDialog.

Managed/themed dialogs that aren't native windows (ThemedAboutDialogHandler, and on WPF after the Xceed removal, ThemedColorDialogHandler / ThemedFontDialogHandler) are made cancellable by simply closing the underlying Eto Dialog — no native interop needed.

Platform status

• GTK — manually tested, works as expected.
• WinForms / WPF — manually tested, works as expected.
• Mac / iOS / Android — preliminary and largely untested; ShowDialogAsync on these should be considered a starting point, not finished.

Not supported

• OpenWithDialog on Windows remains a fire-and-forget shell launch (rundll32 OpenAs_RunDLL), so it doesn't support cancellation. (SHOpenWithDialog was evaluated but on Windows 11 it's a UWP picker that can't distinguish select from cancel, although it does block until the dialog closes.) The async API surfaces this as NotSupportedException, which the test harness catches and logs.