cookbook: Add recipe about sync'ing actor sizes

cookbook: Add recipe about sync'ing actor sizes

The recipe covers how to use ClutterBindConstraint
to bind actor sizes together.

It gives some examples of where this approach is appropriate,
as well as explaining an alternative using allocation-changed
or notify::* signals.

Three examples are given:

1. Resizing a texture to the stage.
2. Resizing a rectangle to act as a transparent overlay on
top of a texture (using constraints).
3. Resizing a rectangle to act as a transparent overlay on
top of a texture, but with a size proportional to the texture
(using a handler connected to allocation-changed signals
emitted by the texture).