st: Make BoxLayout use a Clutter layout manager internally

With the BoxLayout containers in St and Mx and the ClutterBoxLayout
manager, we now have three more or less diverged implementations of
the same layout policy.

While removing StBoxLayout entirely in favor of ClutterLayoutManager
would be the fashionable thing to do, there are obvious drawbacks:

 - it is the only actor we have that implements the scrollable interface
 - it conveniently exposes its spacing property in CSS
 - last but not least, it is used all over the place

So do the next best thing and make our implementation use the
Clutter layout manager internally - that way, the change is
transparent to users, while we get to refer most of the tricky
bits to Clutter. win-win!

https://bugzilla.gnome.org/show_bug.cgi?id=703810
st: Remove empty dispose/finalize methods from StBoxLayoutChild

If the only thing we do is chain up to the parent, there's no
point in not using the parent's implementation directly.

https://bugzilla.gnome.org/show_bug.cgi?id=703810
st: Notify on BoxLayout property changes

We didn't do this before, but it might come in handy eventually,
so add change notifications.

https://bugzilla.gnome.org/show_bug.cgi?id=703810
st: Translate StBoxLayoutChild properties to ClutterBoxChild

With StBoxLayout using the Clutter layout manager, it will now respect
actors' expand/align properties. However for the change to be transparent,
we need to support the existing child meta properties as well. Do
this by simply translating them to the corresponding ClutterBoxChild
properties.

https://bugzilla.gnome.org/show_bug.cgi?id=703810
checkBox: Remove height-for-width hack

With the move to ClutterBoxLayout, height-for-width should now
work properly, so remove this old hack.

https://bugzilla.gnome.org/show_bug.cgi?id=703811
checkBox: Remove custom container implementation

The only point of using a custom container here was to prevent StBoxLayout
from enforcing the wrong request mode based on the orientation. With that
issue fixed, we can simplify the checkbox widget significantly.

https://bugzilla.gnome.org/show_bug.cgi?id=703811
modalDialog: Request HEIGHT_FOR_WIDTH

All our modal dialogs are given a fixed width and grow vertically
as necessary. Set the request mode accordingly, so that wrapped
labels are considered correctly during size request, and not only
at allocation time (where they'll either take away from the padding
or even cause the dialog to overflow).

https://bugzilla.gnome.org/show_bug.cgi?id=704015
networkAgent: Remove height-for-width hack

ClutterBoxLayout has proper height-for-width support, so make use
of it.

https://bugzilla.gnome.org/show_bug.cgi?id=704015