Episerver.Forms.Core should have dependency to CMS.UI.Core only

Description

Forms.Core depends on EPiServer.CMS.UI
A developer installs Episerver.Forms.Core and expects to just get the core files. But since this package has a dependency on Episerver.CMS.UI, the developer also gets all frontend files for CMS in the class libraries.

Why must Forms.Core depend on CMS.UI?

• If part of Forms.Core needs classes in CMS.UI, it should be moved to the Forms.UI.
• It's pointless to split the Forms package into a Core and a UI package.
• We usually keep logic and domain-specific code in separate class libraries.