[FEATURE] Add IsChildPage condition ViewHelper

Condition ViewHelper which renders the `then` child if current page or page with provided UID is a child of some other page in the page tree.
Merge pull request #174 from bjo3rnf/ischildpage

[FEATURE] Add IsChildPage condition ViewHelper
[TASK] Guard against empty/zero page UID passed to pageSelect
Merge pull request #176 from NamelessCoder/pageuid

[TASK] Guard against empty/zero page UID passed to pageSelect