add possibility to overwrite base criteria

experimental, discuss:
• add inst var $sCriteria to SpecializedListWidgetModule.php
• add condition to LinkListWidgetModule::getCriteria (extending above) to check whether a $sCriteria exists
	and include it instead of the local criteria that by default excludes externally managed links
• make some private method protected

Like this I can create for example
• a site/plugin based class ClassLinkListWidgetModule that extends LinkListWidgetModule
• in the ClassDetailWidget the ClassLinkListWidgetModule can be instanciated
• and sets the $iSchoolClassId: ClassLinkListWidgetModule::setClassRoomId() and prepares the $sCriteria

Bug for now
Cannot fetch TableMap for undefined table: class_links
But the related ClassLinkTableMap.php exists?
This issue needs to be tested with the current schulcms plugin master