String freeze request for GNOME Shell

[Florian Müllner <fmuellner gnome org>]

Hey,

sorry for being late on this, but the patch in question depended on a
Mutter change that only landed today. I'd like to commit the remaining
patch in https://bugzilla.gnome.org/show_bug.cgi?id=671010, which will
make the keybinding definitions for the screen recorder and the
message tray show up in the keyboard shortcuts section in System
Settings.

It will add four strings (though for two of them we could extract
existing translations from gnome-control-center):
  "System", "Screenshots" - those are the titles of the sections where
the shortcuts should show up
  "Record a screencast", "Show the message tray" - the new shortcut descriptions

While I don't consider it awfully important to have those shortcuts
configurable, having those entries show up in System Settings will
also advertise the existence of those shortcuts, which I think is
pretty valuable, in particular message-tray one.

Regards,
Florian