Freeze break requests for GTK+ and the network panel

[Matthias Clasen <matthias clasen gmail com>]

We improved the network panel UI recently by converting it to GtkGrid.
This uncovered an embarrassing oversight in GTK+ 3:
The GtkGrid container does not do automatic RTL flipping that people
have come to expect from GTK+ containers. The result is that the
network panel is the sole control-center panel that really falls apart
in RTL locales (see the screenshot in bug 646125).

Adding the flippability to GtkGrid is thankfully a very easy patch,
see bug 646195. To fully fix the network panel, I also need to make
another GTK+ 3 addition RTL-safe, the GtkWidget::halign property.
Doing that requires a minor API addition (two new enumeration values,
since the existing start/end values are explicitly documented in terms
of left/right). Doing this is actually safer than reinterpreting
start/end, since we are guaranteed that the change will only take
effect in the few places where we start using the new values, ie in
the network panel. The patch for this can be seen in bug 646196.

https://bugzilla.gnome.org/show_bug.cgi?id=646125
https://bugzilla.gnome.org/show_bug.cgi?id=646195
https://bugzilla.gnome.org/show_bug.cgi?id=646196

I think these patches carry a very small risk, since they only affect
GTK+ 3 additions, which are not widely used.