XWIKI-13902: Rewrite entities to/fromXML methods based on Filter framework

[Misc] Apply new best practice for@since tags (see http://dev.xwiki.org/xwiki/bin/view/Community/JavaCodeStyle#HUseone40sinceperversion )

XWIKI-13570: When a XAR does not have any extension id associated the package.xml should not containg any <extensionId> element
XWIKI-13569: XAR file reader should ignore empty extension id element

XWIKI-13338: Add ability to override the character symbols used to represent an Entity Reference as a string
* Yet another case of using a component as a POJO, bad....

XWIKI-13338: Add ability to override the character symbols used to represent an Entity Reference as a string
* Fix more tests

XWIKI-13292: Provide a test module to use as dependency for the module manipulating oldcore test tools