Make core fields' has-data check more consistent.

As discussed in #1978.
First refactor for #1572
Only use arrays
Make AjaxPage extends XMLPage
do not redeclare abstract methods
Introduce TextPage, to be more flexible and have some code reuse
Make JSONPage and XMLPage extends TextPage
Clean up comments
Refactored contentAjaxEventDocumentation to use TextPage
Refactored contentAjaxHandle in extends JSONPage
Adapted the js to match the new json output of  contentAjaxHandle
Refactored contentAjaxLog to extends TextPage
Refactored contentAjaxParameters to use JSONPage
Refactored contentAjaxReorder to use XMLPage
Refactored contentAjaxTranslate to use JSONPage
Added a @see TextPage reference
Merge pull request #1979 from michael-e/fix-core-fields

Make core fields' has-data check more consistent. RE: #1978
Merge pull request #1981 from DeuxHuitHuit/ajax-pages

Abstract AJAXPage pages. Closes #1571
Check validation regex on section saving. Fixes #1416
Fix validation check