[MWF] Fix size of text area on message box (Fixes #2090)

Previously some characters might have been concatenated on a message
box that displayed an icon because the size of the text area was
calculated wrong. This fixes bug #2090
(https://bugzilla.xamarin.com/show_bug.cgi?id=2090).
[MWF] Fix MessageBox to wrap texts reasonably

Wrap text in MessageBox window so that the message box has a reasonable
width.  This fixes Xamarin bug #15504.
[MWF] Better account for icon width in MessageBox

This fixes Xamarin bug #16374.