string freeze break for Builder 3.30

[Christian Hergert <christian hergert me>]

On the gnome-builder-3-30 branch, I've included a fix for a scenario
where we cause some bad things to happen in Gtk. To fix this properly, I
had to introduce a couple of new strings.

A new page is displayed while a previous editor session is restored. The
new strings are:

  Restoring previous session
  Your previous session will be ready momentarily.

Thanks for you attention and translations,

-- Christian