filechooserentry: Catch tab key earlier

filechooserentry: Catch tab key earlier

Instead of in the key_press handler, use a signal handler. The signal
handler runs before the default handler, and before other signal
handlers. In particular it runs before the signal handler installed via
gtk_entry_set_completion() which pops down the entry completion, and we
don't want that to happen.

The code does not change the code of the handler in any significant way,
it just refacotrs it to not call the parent anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=663573