#56 Textfield and -dialogui, ESC closes window

[Robert May]

If you press ESC in a textfield that is hosted in a
window that has -dialogui option set, then the window
recieves a WM_CLOSE message and is closed; If this is
a top level window this causes the UI to dissapear, but
leaves the event loop spinning.

To fix, Textfield needs to process WM_GETDLGCODE, and
return 0 for a WM_KEYDOWN message with key VK_ESCAPE.

Current code base makes this difficult to do.