Some fixes in Box2DFixture

* Fixed leaking of memory in case of putting vertices too close
  together, by using a QScopedArrayPointer to simplify cleanup.

* Fixed check on minimum amount of vertices for Chain fixture,
  which should be at least three for a looping chain.

* Some small coding style fixes.
Don't set previous or next vertex for looping Chain fixtures

Box2D docs say to avoid this. For a chain loop, the previous and next
vertices are automatically set as appropriate.

Also fixed the y coordinate of these vertices to be correctly inverted.
Moved vertices for Box fixture back on the stack

There is no reason why these temporarily calculated vertices should be a
member variable. It's faster to have it on the stack.
Improved Box2DBox::createShape

Use b2PolygonShape::SetAsBox, which should be a little more efficient
than setting the box as a polygon since it avoids a bunch of checks and
calculations.

Also, an initial rotation property set on a Box fixture is now
supported, though changes to it are not yet applied.
add possibility to recalculate mass based on fixture densities
add missing QuickItem::geometryChanged() call
re-create fixture shape in the case of position changes
Merge pull request #21 from ElderOrb/resetMassData

add possibility to recalculate mass based on fixture densities
add positionChanged signal to allow drawing trajectory
Merge pull request #20 from ElderOrb/positionChanged

add positionChanged signal to allow drawing trajectory
Merge pull request #24 from ElderOrb/missingGeometryChanged

add missing QuickItem::geometryChanged() call
allow change collision filtering  without fixture re-creation
fix access violation during disposal of nested bodies
Merge pull request #22 from ElderOrb/collisionFiltering

allow change collision filtering  without fixture re-creation
Merge pull request #26 from ElderOrb/accessViolationFix

fix access violation during disposal of nested bodies
Improve DebugDraw item
Fixed positioning support for Circle fixture

The x and y properties of the Circle fixture were being ignored
since change 272a3207fc8e71c67c3a08b3727db057bdecfab6. This change
makes it so that they are being taken into account again, while still
shifting the b2CircleShape to center in the Circle fixture item, at
least when it has its default size.

Instead of setting the width and height explicitly, the implicit
width is now set based on the radius.
expose angularVelocity to QML
Merge pull request #31 from ElderOrb/recreateFixtureShapeOnPositionChanges

re-create fixture shape in the case of position changes
Merge pull request #19 from bjorn/master

Some improvements related to fixtures