[GTK] Toggle OverWrite mode when pressing the Insert key
https://bugs.webkit.org/show_bug.cgi?id=113384

Reviewed by Martin Robinson.

Perform an OverWrite command as a response to the emission of the
toggle-overwrite signal. Due to the nature of the command it will
only do that when being on a richly editable web content.

* platform/gtk/KeyBindingTranslator.cpp:
(WebCore::toggleOverwriteCallback):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147916 268f45cc-cd09-0410-ab3c-d52691b4dbfc