squeezer: Add the xalign and yalign properties

This also centers the children by default instead of aligning them to the start and the top when they are larger than the squeezer, as the default xalign and yalign values are 0.5.

To test it, check the view switcher demo, make the squeezer's speed very slow, and notice how xalign affects the switcher's position as it is fading out to be replaced by the title.

Fixes #226 (closed)