Comment 2 for bug 934083

I'm afraid relying on that flag is not okay. The gtk3 compat code is supposed to mimic upstream.

But swapping set_object_data is the right direction. I went further to set the default text at the top of sokoke_entry_set_default_text. Additionally I implemented gtk_entry_get_placeholder_text so that the clear button callback can see if default text is being used.