Hard code freeze break for gnome-panel for centered clock

[Vincent Untz <vuntz gnome org>]

Hi,

I've pushed a branch to make it possible to have the clock centered by
default:
  http://git.gnome.org/browse/gnome-panel/log/?h=wip/packed-objects
  (starts at "panel: When creating an object, explicitly specify how
   it's packed")
This is for https://bugzilla.gnome.org/show_bug.cgi?id=631553 (minimize
differences between shell & fallback), which is marked as important.

This actually changes a few things to make this possible:
  - change the way applets are positioned: instead of having an absolute
    position for each of them, we now simply pack them at the
    start/center/end of a panel. (Ryan wanted this, and he got it)
  - change the schema, since the old keys were actually not good for
    that. This change means that a user who used gnome-panel 2.91.93
    will have all panel layout broken on update; but it's rather easy to
    fix by just moving the applets again.
  - center the clock by default.

In my tests, it work quite well, and it's actually useful to help the
users keep a clean panel. It also fixes a bunch of bugs where the
applets were not at the right place after a screen resize, or after
moving a panel.

There are some unrelated fixes in the branch, though. They'd be nice to
have, but not strictly necessary:

  - Apply delayed GSettings before unref'ing it to not lose change:
    http://git.gnome.org/browse/gnome-panel/commit/?h=wip/packed-objects&id=75fe30f62bb06c4afda64857207710dd88483224

  - Add ranges to schemas to avoid invalid settings:
    http://git.gnome.org/browse/gnome-panel/commit/?h=wip/packed-objects&id=ea08f1b0dcdd77c209150871ad2e957b06c3531e

Overall, this makes the fallback mode look much closer to gnome-shell,
which is quite nice.

FWIW, doc team was fine with a change like this, and there's no string
change involved.

Opinions?

(Working on the status menu now, so I might have another freeze break
request later on)

Vincent

-- 
Les gens heureux ne sont pas pressés.