[TASK] Refactor reading of pid in AbstractContentViewHelper

Method has been created to make a uniform way of getting the currently configured or auto-detected PID value to use when reading content records.
[TASK] Remove double variable declaration

Cosmetic effect only.
[TASK] Ship XSD schema file
[BUGFIX] PageSelect root line cache key ignores reverse instruction
[TASK] Clean up logic of AbstractContentViewHelper
[BUGFIX] Incorrect use of grouping in content fetching

Symptom: renders only one content element from each column.
[TASK] Add missing attribute pageUid on page.menu.browse
[BUGFIX] Correct targetNamespace in XSD schema