prepareAssociationsDrawerXMLElement

make child associations call
$field->prepareAssociationsDrawerXMLElement just like parents do.
Added the missing prepopulate value

This is required in order to make the function compatible with both
child and parent relationships.
Fallback value for DrawerXMLElement

Since prepareAssociationsDrawerXMLElement is kind of
new, 99% of the fields do not implement it. Instead, they
rely on their prepareTableValue implementation to make
association drawer work. But, drawer now directly calls
preparePlainTextValue, some field were displaying nothing.
This fix actually calls prepareTableValue if the plainTextValue
is empty. This could be removed in future versions.
Properly format prepopulate doc
Merge pull request #2101 from DeuxHuitHuit/issue-2085

Allows pre-populating with the Associations Drawer. Closes #2085