st-box-layout: Pass correct allocation box to layout manager

st-box-layout: Pass correct allocation box to layout manager

StBoxLayout implements StScrollable, which, semantically, means that
the StBoxLayout size may not match the minimum size reported by the
layout manager. In this specific case, the layout manager by is a
ClutterBoxLayout by default. For example:

        +--------------+
        |   Viewport   |
 +------+--------------+-----------------+
 |      |              |                 |
 |      |              |     Content     |
 |      |              |                 |
 +------+--------------+-----------------+
        |              |
        +--------------+

So, assuming that:

 - ContentSize = the minimum size of the content;
 - ViewportSize = the allocated size of the viewport;

When allocating StBoxLayout, it must assume ViewportSize, but must
pass ContentSize to the layout manager. That way, the children of
StBoxLayout are correctly placed within it, even if it's bigger than
ViewportSize.

And here's the problem: right now, StBoxLayout assumes ViewportSize
AND also passes it to layout manager. Commit 77c4c6b6d specifically
exposed this bug by relying entirely on StBoxLayout to arrange the
app and window icons.

Fix that by using ViewportSize to allocate StBoxLayout itself, but
passing ContentSize to the layout manager.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/153