#2810 tk_messageBox can return empty string on Windows

[Eric Boudaillier]

If the parent window of a tk_messageBox is transient, and the window is iconified, the message box disappears and returns the empty string.

# Example;
package require Tk

tk_messageBox -message test
# Iconify the application, the message dialog stay open

toplevel .t
tk_messageBox -parent .t -message test
# Iconify the application, the message dialog stay open

wm transient .t .
tk_messageBox -parent .t -message test
# Iconify the application, the message dialog disappears
# Deiconify the application, tk_messageBox returns empty string