Split the retention starting point properties

Waiting for reworking an event-based retention management, we want to hide the EL capability.

To do so, we want to change the existing label in the Retention period starting point menu (cf. attached screenshot):

• current label: "Input (or EL Condition)"
• new label: "Event input"

Also to avoid the server errors if EL is not valid (because initially the same property is used for EL here and not EL here ), we take the choice to split the property into two separate ones:

• retention_def:startingPointExpression to handle the EL, used here
• retention_def:startingPointValue to handle the no EL value here

For moment we add this possibility into the back end part. and only use the retention_def:startingPointValue in the front end, for EL the needed front end par will be added later.