Update the simple dialog code.

Rename simple_dialog_qt.{cpp,h} to simple_dialog.{cpp,h}. Make it a
subclass of QMessageBox. Queue messages at startup similar to GTK+.

Move the GTK+-specific simple_dialog declarations to
gtk/simple_dialog.h.

Don't yell at the user so much. Replace exclamation points with periods.

Change-Id: I1cc771106222d5e06f1f52d67ac29d6dc367cce4
Reviewed-on: https://code.wireshark.org/review/4288
Reviewed-by: Gerald Combs <gerald@wireshark.org>