Support multi-pick/value custom property (settings UI)

Support for multi-select lists for custom properties.

In settings for custom properties it should be possible to set a custom property having a "multi-select" type, and to define the pick list which will be used in the select
This is an amendment to the existing forms in Settings for Agreements & Licenses
If the "multi-select" type is chosen, the user must set a pick list (as currently works for the "Pick List" (aka Refdata) property type)