popup: nested popup doesn't consider zIndexGroupLevel of parent popup (closes #970)

closes #970
I'm not sure about addition of zIndexes. For instance, we have three nested popups with `zIndexGroupLevel: 1`. The third popup will calculate his `_zIndexGroupLevel` as `3`. Then we place another popup in block `z-index-group_level_2` somewhere on the page. Now the third popup will overlap last one. Looks like a bug, because in CSS z-indexes the last popup will overlap the third popup.

btw, i think we shouldn't call `_calcZIndexGroupLevel()` on parent block, because `_zIndexGroupLevel` should be calculated  only when popup shows or changes owner. In other cases `_zIndexGroupLevel = null`, which casts to 0 in `Math.max()`.