Improve performance of Category_Generator

Create parent lazily using existing _lazyParent feature.

This avoids deep stack nesting level when having many levels of pages in the page tree. It fixes an segfault with Php 5.2 starting from level 5 when loading page properties (for Porsche)
remove selfVisible and replace it with visible

field that also contains visiblity of parent pages is now parent_visible, as it was initially intended (but buggy implemented).

Other table generators behave the same (visible can be true although parent is invisible)

fixes tests that where broken with lazy parent as it's visiblity was not checked correctly
Merge pull request #232 from nsams/less-stack-depth

Pages Generator Improvements: Less stack depth