[layout] Add layers to BinLayout

[layout] Add layers to BinLayout

Each actor managed by a BinLayout policy should reside inside its
own "layer", with horizontal and vertical alignment. The :x-align
and :y-align properties of the BinLayout are the default alignment
policies, which are copied to each new "layer" when it is created.

The set_alignment() and get_alignment() methods of BinLayout can
be changed to operate on a specific "layer".

The whole machinery uses the new ChildMeta support inside the
LayoutManager base abstract class.