Based World, Body and Fixture on QObject instead of QQuickItem

This gives much more flexibility and less overhead.

It also gets rid of the automatic discovery of bodies to animate.
However, this may not be as inconvenient as it seems.

Only 'demolition' example adjusted so far.
Introduced BoxBody, CircleBody, PolygonBody, etc. helpers to examples

These make notation slightly shorter in the common case where one
fixture suffices for a body.

Only used in the 'demolition' example for now.
Introduced ImageBoxBody

Not exactly sure yet whether this convenience class is worth it. The
use-case may be common but is also very limited.
Made Body.fixtures the default property

This wasn't possible before when Body was a subclass of Item.
It avoids having to write "fixtures: [...]"
Keep track of synchronization in World instead of Body

Saves some bytes and maybe a few cycles.
Raised API version to 2.0

Because the change to QObject broke backwards compatibility.
Create the body also when no target is set

It is not actually necessary for a body to have a target for it to be
relevant. Creating the body also when no target is set allows for
light-weight obstacle and sensor definitions (without visual
representation).
Adjusted all examples to the QObject-based API

The main thing here is that the World, Body and fixture components are
no longer derived from Item but from QObject. This means they don't have
a parent item and they don't have anchors.

Many examples now use a shared PhysicsItem component that is similar to
the previous Body component, in that it combines an item with a Body
instance that has the item as its target.
Merge pull request #70 from bjorn/qobject

Base World, Body and Fixture on QObject instead of QQuickItem