[TASK] Move registration of PageConfigurationProvider to ext_localconf.php

Apparently, 6.0 requires it be placed here in order to be recognised in the FE.
[FEATURE] Proper page settings inheritance

This adds inheritance of page variables for sub pages using either an inherited page template or the same template as the parent page. In addition a new approach to trim inherited variables has been implemented, greatly improving the predictability - changing a variable in a parent page makes that variable effective on any page down the rootline until either the page template is changed or the variable in question has been intentionally overridden by a page.
[TASK] Add built-in, blank error page necessary to avoid errors
[FEATURE] Special error page

In addition, some added error handling is implemented and Flux's debug switch is enabled in contexts where Flux applies but Flux itself may not be processing the thrown Exception correctly.
[TASK] Exclude disabled page templates, implement level 2 cache
[BUGFIX] Restore manual setting of filename

This template filename assignment had previously been removed while working to add a special, display-friendly error action.

Fixes: #8
[BUGFIX] Prevent case of endless loop redirection in page/list module
[TASK] Remove unused variable assignments in PageController

Fixes: #8
[TASK] CGL cleanup to 100% compliance according to metrics.typo3.org
[TER] 1.2.0