project: Add the aspect_ratio dynamic property
timeline: Avoid calling a callback method directly
timeline: Remove unuseful perspective logic
Do not set the z position for timeline elements

Setting the z position changes the x and y of the element, we don't want
that.
timeline: Get rid of method used in a single place
timeline: Test directly the element, not its parent
timeline: Be more strict when a timeline element is added
tests: Allow the tests to be run by frameworks like nosetests