Fix really bad modelfilter bug that may have caused segfaults.
This also fixes the problem of showing the wrong icon when expanding metacontacts.
Example:
We have the groups A, B, C in our child model
But only A and C are visible (and therefore in modelfilter)
Imagine we have a childIter_G pointing to group B
Getting its path in the childmodel, will give us (lets say) 2
2 means now the second group/row in the CHILD model
We now pass this to the tree and tell him "Expand row 2"
The tree itself is looking at the 2nd row of modelfilter: "Hey modelfilter, expand row number 2"
For modelfilter row number 2 is C and NOT B