We are using a checkbox list property for selection multiple items from a list of items. The problem is that it contains over 100 items and becomes extremely long in the UI. We want to replace this with an editor which uses two select lists. All items are available on the left side then you can pick the ones you need and move them to the select list on the right side. Does this exist in episerver or has anyone done anything like that before? Otherwise could someone point to some tutorial or documentation for building it ourselves?
To the best of my knowledge, this property editor doesn't exist. I believe there was something built in Episerver 6 that did this (similar to the administrative users interface), but I haven't seen anything in 7+.
My recommendation is to implement a custom EditorDescriptor that displays your existing SelectMany fields in the interface that you are describing. You'd do this via Dojo. You would then use a UiHint to trigger the editor interface to use your alternate style. With this approach, your existing data stores and property values would remain working.
This article is a very in-depth walkthrough to doing exactly what I've described: http://www.patrickvankleef.com/2015/02/03/episerver-custom-property-in-dojo/
Before you get started - I recommend reading the "User Interface" section of the Developer Guides as Dojo has a bit of a learning curve:
Hope this helps,