Allow using a different name for the static import

QML currently doesn't allow doing qmlRegisterType on a plugin URI
already defined in an installed plugin. It makes sense to use a
different URI for the box2d import when linking it statically to
avoid a fatal QML load error on systems with this plugin installed.

Example:
    Box2DPlugin plugin;
    plugin.registerTypes("Box2DStatic");

This reuses the STATIC_PLUGIN_BOX2D define in box2dplugin.h and
define it properly in box2d-static.pri. The assert is only checked
on non-static builds.
Update bodies after changing pixelsPerMeter

Only active dynamic bodies are currently updated during the following
world step.

This patch forces an update of body and fixture underlying meter
metrics according to their target's current pixel values but using
the new pixelsPerMeter value for the conversion.

It's then the responsibility of the application to make sure that
bodies and fixtures using meter coordinates (i.e. values multiplied
by World.pixelsPerMeter) are updated before the next world step
using property bindings or an onPixelsPerMeterChanged handler.

This follows fixes for issue #28.