PresetDialog: Select text cell of freshly-added status

PresetDialog: Select text cell of freshly-added status

This means you can just mash Enter to edit your mistake. It's more
useful than leaving the text entry focused. It also helps hide a bug
where the entry doesn't get cleared if you just start typing another
message. So previously, typing:

  Hello<Enter>Miaow<Enter>

would give you two statuses, "Hello" and "MiaowEnter Custom Message".

It didn't seem worth fixing this much more since Gtk+ 3.2 will probably
grow native support for placeholder text:
<https://bugzilla.gnome.org/show_bug.cgi?id=440963>