Fix marking of prototype objects in internal class pool

As per reported bug, we have to protect ourselves against potential loops
and can mark the internal classes much simpler by just walking through
the memory pool they were allocated in.

Task-number: QTBUG-38299
Change-Id: I3ae96e8082e76d06f4321c5aa6d2e9645d2830a0
Merge pull request #14 from mbrasser-jolla/mer-stable

[qtdeclarative] Fix marking of prototype objects in internal class pool