Fixed memory leak in contact listener and uninitalized value in box2dbod...

Hi gals and guys,
valgrind complains about two issues in the latest qml-box2d.
1)
Uninitalized value mComponentComplete in:
<code>
void Box2DBody::createBody()
{
    if (!mWorld)
        return;

    if (!mComponentComplete) {
</code>

2) Memory leak in Contact Listenener:

<code>
void Box2DWorld::componentComplete()
{
    mComponentComplete = true;

    enableContactListener(mEnableContactEvents);
</code>

calls
<code>
mContactListener = new ContactListener(this);
</code>

and memory does  nor get released.

Best regards,
Thomas
Please list `mComponentComplete` before `mTransformDirty` to avoid compiler warnings about the initialization order (which in this case will be different from the order you've written it down in).