Use a QPersistantModelIndex instead and fetch the item only when it's
needed. This way we can make sure that we don't crash if the item was
deleted during the job's life.

This is a bug fix, but I'm only committing to master for the moment.

BUG: 319335
CCBUG: 293191

Conflicts:
	projectbuilders/makebuilder/makejob.cpp
	projectbuilders/makebuilder/makejob.h