CMS-3897 Make input type Date
Added support for Value having a null value. This is needed for value types not being string-based. Otherwise inputs using such value types cannot have "no value".
Added several TS ValueTypes classes to support this.
Overhauled several TS input type classes to support this.
Changed input type Image to extend BaseInputTypeNotManagingAdd, simplyfing it's code and removing some bugs.