TYPO3 Core

We can only use 3 constants in TCA foreign_table_where : PAGE_TSCONFIG_ID, PAGE_TSCONFIG_IDLIST, PAGE_TSCONFIG_STR

Add custom constants could be cool. Two use cases:
- complex where condition
- in the flexforms, the field is always tt_content.pi_flexform

The constant names could be : PAGE_TSCONFIG_CUSTOM_(ID|IDLIST|STR)_constantName

You can show alpha patch 6.2 for this feature : (attachment constant_v0.1.diff)

To do :
- same logique for suggest in TYPO3\CMS\Backend\Form\Element\SuggestElement
- add test
- update documentation