[qmozcontext] Remove redundant autoInit attribute from QmlMozContext

Scheduling QMozContext.runEmbeding to next event loop iteration in
the constructor of QMozContext guaranties that gecko startup
will happen in the next iteration. Thus the attribute autoInit in
QMozContext::GetInstance() and explicit call to
QMozContext::GetInstance()->runEmbedding(0) before
starting Qt event loop are redundant.
[qmozcontext] Drop QmlMozContext in favour of global QMozContext instance

QmlMozContext is a declarative workaround for QMozContext singleton.
Until there is a better support for singletons in QML make an instance
of QMozContext to be global object in QML.
Semantically a global instance makes clearer suggestion that it's
a singleton than an attribute of a QML component.
[qmozcontext] Avoid QThread subclassing

Starting from Qt 4.4 QThread subclassing is considered to be
a bad practice: a QThread should only manage a thread, not process
data.
Setup embedding explicitly in main()
Merge pull request #13 from rojkov/cleanup1

Make sematics of QMozContext clearer.