QmlDesigner: Fix crash when opening a .qml file with states

This fixes an assert in QGraphicsScene because of
overlapping deferred calls of _q_processDirtyItems & _q_emitUpdated.
Both used to be queued in the same event loop:

 _q_emitUpdated call queued with "calledEmitUpdated=true"
 _q_processDirtyItems call queued
-> Event loop runs
 _q_emitUpdated runs, setting "calledEmitUpdated=false"
 _q_processDirtyItems runs, asserting because of !calledEmitUpdated

This fix forces the event loop to run between the queueing of
_q_emitUpdated & _q_processDirtyItems, effectively separating
both calls.

Task-number: BAUHAUS-459
Reviewed-by: Christiaan Janssen