MessageBox returns the pressed button and supports multiple buttons

The MessageBox(message) global function currently shows a dialog with a single OK button and returns no value. The MessageDialog class already supports up to three custom-caption buttons and reports which one was pressed via Show(), but the quick MessageBox form cannot.

Requested change:

• MessageBox returns a MessageDialogButton (Action, Alternate, or Cancel) identifying the button the user pressed. The one-argument form shows only OK and therefore always returns MessageDialogButton.Action.
• New overloads allow a multi-button message box in a single call, accepting captions for the action, cancel, and alternate buttons instead of requiring a configured MessageDialog instance.

Existing code that calls MessageBox("...") as a statement continues to compile unchanged; the return value is purely additive. The web framework keeps its existing non-blocking MessageDialog with the Dismissed event; MessageBox remains desktop-only.